gdb: remove inline_frame::skipped_frames

While reviewing [1], it occurred to me that having both the
skipped_frames counter and the skipped_syms vector is redundant.  When
stepping into an inline frame, we can just pop the last element.

[1] https://inbox.sourceware.org/gdb-patches/96cfee31-6a50-4a78-a25b-67e5d061c2a3@simark.ca/T/#m7e0e4b5b6cfc91be3d8ab6d5025a97c2e647103a

Change-Id: I8c10e7fcd05e41c2c838431d06c9e793d18a2198
Approved-By: Andrew Burgess <aburgess@redhat.com>