Remove unused Linux libthread_db callbacks

gdb/proc-service.c includes several libthread_db callbacks that do not
exist in gdb/gdbserver/proc-service.c.  Other than in proc_service.h,
there is no reference to any of these callbacks in any revision of
nptl_db or linuxthreads_db in glibc's git repo so it seems likely that
these functions have never been called.  This commit removes them.

gdb/
2014-07-01  Gary Benson  <gbenson@redhat.com>

	* proc-service.c (ps_xfer_memory): Update comment.
	(ps_pstop): Remove unused function.
	(ps_pcontinue): Likewise.
	(ps_lstop): Likewise.
	(ps_lcontinue): Likewise.
	(ps_lgetxregsize): Likewise.
	(ps_lgetxregs): Likewise.
	(ps_lsetxregs): Likewise.
	(ps_plog): Likewise.
	(ps_ptread): Likewise.
	(ps_ptwrite): Likewise.