Remove some cleanups from tracepoint.c

Remove some cleanups from tracepoint.c

This removes some cleanups from tracepoint.c by using std::string.  It
also removes some unused cleanup declarations.

gdb/ChangeLog
2017-09-29  Tom Tromey  <tom@tromey.com>

	* tracepoint.c (trace_variable_command): Use std::string.
	(encode_actions_1): Remove unused declarations.
	(create_tsv_from_upload): Use std::string.