This removes the interp_name function.  It is only used a few spots --
one of which was only calling it on "this".  It's simpler to remove
it; and should class interp become opaque in the future, it will be
just as easy to update the two remaining spots to use an accessor.

ChangeLog
2018-05-25  Tom Tromey	<tom@tromey.com>

	* interps.c (interp_name): Remove.
	* mi/mi-interp.c (mi_interp::init): Update.
	* interps.h (interp_name): Remove.
	(~scoped_restore_interp): Update.
	* tui/tui.c (tui_enable): Update.