gdb/python: fix a few flake8 warnings

gdb/python: fix a few flake8 warnings

Fix these rather obvious warnings reported by flake8:

    ./lib/gdb/FrameIterator.py:16:1: F401 'gdb' imported but unused
    ./lib/gdb/FrameIterator.py:17:1: F401 'itertools' imported but unused
    ./lib/gdb/command/prompt.py:55:26: E712 comparison to False should be 'if cond is False:' or 'if not cond:'
    ./lib/gdb/command/explore.py:526:9: F841 local variable 'has_explorable_fields' is assigned to but never used
    ./lib/gdb/command/explore.py:697:56: E712 comparison to False should be 'if cond is False:' or 'if not cond:'
    ./lib/gdb/command/explore.py:736:62: E712 comparison to False should be 'if cond is False:' or 'if not cond:'
    ./lib/gdb/command/explore.py:767:61: E712 comparison to False should be 'if cond is False:' or 'if not cond:'
    ./lib/gdb/command/frame_filters.py:21:1: F401 'copy' imported but unused
    ./lib/gdb/command/frame_filters.py:22:1: F401 'gdb.FrameIterator.FrameIterator' imported but unused
    ./lib/gdb/command/frame_filters.py:23:1: F401 'gdb.FrameDecorator.FrameDecorator' imported but unused
    ./lib/gdb/command/frame_filters.py:25:1: F401 'itertools' imported but unused
    ./lib/gdb/command/frame_filters.py:179:17: E712 comparison to True should be 'if cond is True:' or 'if cond:'

Change-Id: I4f49c0cb430359ee872222600c61d9c5283b09ab