remote & target_extra_thread_info, use cache w/ qThreadExtraInfo and qP too

The following patch will make "info threads" call target_extra_thread_info
more frequently.  When I looked at the remote implementation, I noticed
that if we're not using qXfer:threads:read, then we'd be increasing the
remote protocol traffic.  This commit prevents that from happening.

Also, it removes a gratuitous local static buffer, which seems good on
its own.

gdb/ChangeLog:
2018-06-29  Pedro Alves  <palves@redhat.com>

	* remote.c (remote_target::extra_thread_info): Delete
	'display_buf' and 'n' locals.  from the cache, regardless of
	packet mechanims is in use.  Use cache for qThreadExtra and qP
	methods too.