Remove unused TASKKILL fallback in AutoInterrupt

When Git.AutoInterrupt was first implemented in ead94f2, it used
os.kill (sending SIGINT to the process to terminate it). This was
in 2009, and not all supported versions of Python provided os.kill
on Windows, since it was added for Windows in 2.7 and 3.2 (2.6 and
3.1 reached EoL in 2013 and 2012, respectively). Catching
AttributeError and running TASKKILL provided a fallback for Python
versions in which os.kill was absent on Windows.

Since then, all supported versions have os.kill on Windows, and
also the code of GitPython, in the try-block, has changed, and no
longer uses os.kill. It now contains four attribute access
expressions:

- proc.terminate. All currently suppported versions of Python
  (including 3.7, which GitPython still supports, and some before
  that) have Popen.terminate.

- proc.wait. Same situation as proc.terminate. Popen.wait exists.

- self._status_code_if_terminate. This is a class attribute of
  AutoInterrupt, accessible through its instances.

- self.status. This is assigned to. AutoInterrupt is slotted, but
  "status" appears in __slots__, so this isn't an AttributeError
  either.

The "except AttributeError" clause is thus no longer used and can
be removed, which is done here. Removing it shortens and simplifies
the code, better expresses the logic for how the wrapped process is
actually being terminated, and eliminates the need to engage with
any subtleties of TASKKILL (and of how it is called) when reading
the code to verify its correctness.

In addition, because they are now expected always to be available,
if somehow an AttributeError managed to be raised in the terminate
or wait calls, this would be very strange and we probably shouldn't
silently catch that.

(Because this AutoInterrupt._terminate method is sometimes called
due to __del__ methods running as the interpreter is shutting down,
it is possible for some attributes to unexpectedly be set to None,
or perhaps on some implementations, absent. The _terminate method
handles this where relevant. But the TASKKILL fallback removed here
seems unrelated to that, which affects module attributes and global
variables. The names used in the try-block are proc, status, and
self, which are local variables; the except clause itself accessed
os.name, which used a global variable and a module attribute,
indicating that the intent was not to handle this interpreter
shutdown scenario; and this whole issue, while not gone completely,
is much less significant since Python 3.4, due to
https://docs.python.org/3/whatsnew/3.4.html#whatsnew-pep-442.)

git/cmd.py

@@ -11,7 +11,7 @@
 import logging
 import os
 import signal
-from subprocess import call, Popen, PIPE, DEVNULL
+from subprocess import Popen, PIPE, DEVNULL
 import subprocess
 import threading
 from textwrap import dedent
@@ -544,16 +544,6 @@ def _terminate(self) -> None:
                 self.status = self._status_code_if_terminate or status
             except OSError as ex:
                 log.info("Ignored error after process had died: %r", ex)
-            except AttributeError:
-                # Try Windows.
-                # For some reason, providing None for stdout/stderr still prints something. This is why
-                # we simply use the shell and redirect to nul. Slower than CreateProcess. The question
-                # is whether we really want to see all these messages. It's annoying no matter what.
-                if os.name == "nt":
-                    call(
-                        ("TASKKILL /F /T /PID %s 2>nul 1>nul" % str(proc.pid)),
-                        shell=True,
-                    )
             # END exception handling
 
         def __del__(self) -> None: