gdb: remove duplicate declaration of 'find_thread_ptid'

gdb: remove duplicate declaration of 'find_thread_ptid'

There are two declarations of 'find_thread_ptid' in gdbthread.h
with the same signature:

  /* Find (non-exited) thread PTID of inferior INF.  */
  extern thread_info *find_thread_ptid (inferior *inf, ptid_t ptid);

and

  /* Search function to lookup a (non-exited) thread by 'ptid'.  Only
     searches in threads of INF.  */
  extern struct thread_info *find_thread_ptid (inferior *inf, ptid_t ptid);

Retain the former, remove the latter.  Tested by rebuilding.

gdb/ChangeLog:
2021-06-29  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* gdbthread.h (find_thread_ptid): Remove the duplicate declaration.