Revert "libcxx: "Fix" shallow runtimes build for clones"

This is insufficient in Jenkins, where that commit happens to be present
but HEAD is shallow (at least on ctsrd-build-freebsd-f1), meaning we
don't hit the invalid commit case and instead end up thinking we know
it's not an ancestor. Moreover, in the case where you have a non-shallow
old checkout, that commit won't exist, yet we now treat it as True,
which is also wrong. Really contains_commit needs to get a lot more
complex to support handling these different cases with different
fallback values, but even then it won't work very well.

This reverts commit a8e1870.

pycheribuild/projects/cross/libcxx.py

@@ -600,8 +600,7 @@ def configure(self, **kwargs) -> None:
                 self.add_cmake_options(LIBCXX_ENABLE_ASSERTIONS=True)
                 # Need to export the symbols from debug.cpp to allow linking code that defines _LIBCPP_DEBUG=1
                 self.add_cmake_options(LIBCXX_ENABLE_BACKWARDS_COMPATIBILITY_DEBUG_MODE_SYMBOLS=True)
-        if not GitRepository.contains_commit(self, "f4fdc4f4d9f6903808541645d383be2ee759f400", src_dir=self.source_dir,
-                                             invalid_commit_ref_result=True):
+        if not GitRepository.contains_commit(self, "f4fdc4f4d9f6903808541645d383be2ee759f400", src_dir=self.source_dir):
             self.fatal(f"LLVM revision in {self.source_dir} is too old for this target, please update.")
         super().configure(**kwargs)