btrace: Remove VEC cleanups

This patch replaces two VEC(tp_t) with std::vector<thread_info *>, which
allows to remove two cleanups.  To make it easier to map the old code to
the new code, I added the ordered_remove and unordered_remove functions,
which operate on std::vector and do the same as VEC's
ordered_remove/unordered_remove.

gdb/ChangeLog:

	* record-btrace.c (record_btrace_maybe_mark_async_event): Change
	parameter types to std::vector.  Use bool.
	(record_btrace_wait): Replace VEC(tp_t) with
	std::vector<thread_info *>.
	* common/gdb_vecs.h (unordered_remove, ordered_remove): New.