gdb/remote.c: Eliminate target_buf/target_buf_size hack

gdb/remote.c: Eliminate target_buf/target_buf_size hack

This finally eliminates an old hack left in place when tracepoint RSP
support was migrated from tracepoint.c to remote.c, back in
35b1e5cca081 ("Make tracepoint operations go through target vector.")
over 7 years ago.

Tested on x86_64 GNU/Linux.

gdb/ChangeLog:
2017-09-29  Pedro Alves  <palves@redhat.com>

	* remote.c (target_buf, target_buf_size): Delete.
	(remote_get_noisy_reply): Remove buf_p and sizeof_buf parameters.
	Use the connection's packet buffer instead.
	All callers adjusted.
	(_initialize_remote): Remove references to target_buf and
	target_buf_size.