Remove output_command_const

Remove output_command_const

I happened to notice that output_command_const still exists, but is
not needed any more -- commands are always const-correct now.  This
patch removes this leftover.

2018-05-21  Tom Tromey  <tom@tromey.com>

	* printcmd.c (output_command): Remove.
	(output_command_const): Rename to output_command.
	* valprint.h (output_command): Rename from output_command_const.
	* tracepoint.c (trace_dump_actions): Call output_command.