refactor: remove info on which macros reserved an object

`MacroExecutor` maintains two data structures:
- `_reserved_macro_objs` - which keeps list of reserved objects
  per macro
- `_reserved_objs` - which keeps list of reserving macros
  per object
While `_reserved_macro_objs` is used when stopping a macro,
`_reserved_objs` is not used anywhere. Remove `_reserved_objs`
data structure cause it stores objects as keys what this forces
them to be hashable and we would like to implement equality check
in the `Motion` class (one type of possibly reservable objects).

src/sardana/macroserver/msmacromanager.py

@@ -1068,11 +1068,6 @@ def __init__(self, door):
         self._door = door
         self._macro_counter = 0
 
-        # dict<PoolElement, set<Macro>>
-        # key PoolElement - reserved object
-        # value set<Macro> macros that reserved the object
-        self._reserved_objs = {}
-
         # dict<Macro, seq<PoolElement>>
         # key Macro - macro object
         # value - sequence of reserverd objects by the macro
@@ -1551,11 +1546,6 @@ def run(self, params, asynch=True):
                        macro script
         :return: (lxml.etree.Element) the xml representation of the running macro
         """
-        # dict<PoolElement, set<Macro>>
-        # key PoolElement - reserved object
-        # value set<Macro> macros that reserved the object
-        self._reserved_objs = {}
-
         # dict<Macro, seq<PoolElement>>
         # key Macro - macro object
         # value - sequence of reserved objects by the macro
@@ -1823,10 +1813,6 @@ def reserveObj(self, obj, macro_obj, priority=0):
             else:
                 objs.append(obj)
 
-        # Fill _reserved_objs
-        macros = self._reserved_objs[obj] = self._reserved_objs.get(obj, set())
-        macros.add(macro_obj)
-
         # Tell the object that it is reserved by a new macro
         if hasattr(obj, 'reserve'):
             obj.reserve(macro_obj)
@@ -1858,11 +1844,3 @@ def returnObj(self, obj, macro_obj):
         objs.remove(obj)
         if len(objs) == 0:
             del self._reserved_macro_objs[macro_obj]
-
-        try:
-            macros = self._reserved_objs[obj]
-            macros.remove(macro_obj)
-            if not len(macros):
-                del self._reserved_objs[obj]
-        except KeyError:
-            self.debug("Unexpected KeyError trying to remove reserved object")