This removes ptid_is_pid in favor of the ptid_t::is_pid method.

gdb/ChangeLog
2018-07-03  Tom Tromey  <tom@tromey.com>

	* common/ptid.c (ptid_is_pid): Remove.
	* common/ptid.h (ptid_is_pid): Don't declare.
	* infrun.c: Update.
	* linux-nat.c: Update.
	* mi/mi-interp.c: Update.
	* remote.c: Update.
	* thread.c: Update.

gdb/gdbserver/ChangeLog
2018-07-03  Tom Tromey  <tom@tromey.com>

	* linux-low.c: Update.