Remove a use of print_expression

Remove a use of print_expression

The tracepoint code uses print_expression to reconstruct an expression
string.  However, the original expression is already available -- it
was just parsed a bit earlier in the same function.  This patch
changes this code to simply save the already-parsed expression, rather
than attempt to reconstruct it.

gdb/ChangeLog
2021-01-09  Tom Tromey  <tom@tromey.com>

	* tracepoint.h (class collection_list) <append_exp>: Take a
	std::string.
	* tracepoint.c (collection_list::append_exp): Take a std::string.
	(encode_actions_1): Update.