Remove in_user_command

Remove in_user_command

While working on the next patch in this series, I found that the
global in_user_command is not used.  This patch removes it.  (I didn't
think to check Insight until submitting this series; and it's not very
convenient to do so, so if someone has it checked out and could look
at it, that would be nice.)

ChangeLog
2017-08-03  Tom Tromey  <tom@tromey.com>

	* top.h (in_user_command): Remove.
	* top.c (in_user_command): Remove.
	* cli/cli-script.c (do_restore_user_call_depth)
	(execute_user_command): Update.