btrace: Remove btrace disable cleanup

This patch removes a cleanup that disables btrace on threads in case of
failure, so we don't leave it enabled for some the threads and disabled
for the rest.

gdb/ChangeLog:

	* record-btrace.c (record_btrace_disable_callback): Remove.
	(struct scoped_btrace_disable): New.
	(record_btrace_open): Use scoped_btrace_disable.