Make encode_actions_rsp use std::vector

Make encode_actions_rsp use std::vector

Currently, encode_actions_rsp returns two malloc'ed arrays of malloc'ed
strings (char *) by pointer.  Change this to use
std::vector<std::string>.  This eliminates some cleanups in remote.c.

Regtested on the buildbot.

gdb/ChangeLog:

	* tracepoint.h (class collection_list) <stringify>: Return
	std::vector<std::string>.
	(encode_actions_rsp): Change parameters to
	std::vector<std::string> *.
	* tracepoint.c (collection_list::stringify): Return
	std::vector<std::string> and adjust accordingly.
	(encode_actions_rsp): Changee parameters to
	std::vector<std::string> and adjust accordingly.
	* remote.c (free_actions_list),
	free_actions_list_cleanup_wrapper): Remove.
	(remote_download_tracepoint): Adjust to std::vector.