[binutils-gdb] Remove unused variable in record-btrace.c

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 8ec235834d454a66a89fe16bc62e11497eb79078 ***

Author: Simon Marchi <simon.marchi@ericsson.com>
Branch: master
Commit: 8ec235834d454a66a89fe16bc62e11497eb79078

Remove unused variable in record-btrace.c

old_inferior_ptid is unused, this is caught by a gcc built from git
recently, not sure about previous versions:

/home/emaisin/src/binutils-gdb/gdb/record-btrace.c: In function frame_info* get_thread_current_frame(thread_info*):
/home/emaisin/src/binutils-gdb/gdb/record-btrace.c:1974:10: error: unused variable old_inferior_ptid [-Werror=unused-variable]
1974 |   ptid_t old_inferior_ptid;
     |          ^~~~~~~~~~~~~~~~~

gdb/ChangeLog:

	* record-btrace.c (get_thread_current_frame): Remove
	old_inferior_ptid.