gdb/testsuite: remove unused but set variables in Python files

gdb/testsuite: remove unused but set variables in Python files

Fix flake8 errors like this one:

    gdb/testsuite/gdb.python/py-unwind-inline.py:61:17: F841 local variable 'v' is assigned to but never used

For this one, there is a slight possibility that removing a variable
changes some behavior, and perhaps renders some test uneffective (where
the test would no longer exercises what it meant to).  Removing a
variable means that the object it pointed to likely gets de-allocated
earlier (its tp_dealloc method gets called).  So if the intent of the
variable was to ensure the variable outlives the statements that come
after, then this change would be wrong.  But I didn't see any evidence
of that being the intent in all the occurences.

Change-Id: Ic57bc68ad225a43ae6771c47b7f443956e8029a6
Approved-By: Tom Tromey <tom@tromey.com>