gdb smob cleanups

	* guile/guile-internal.h (GDB_SMOB_HEAD): Replace properties with
	empty_base_class.  All uses updated.
	(gdbscm_mark_gsmob, gdbscm_mark_chained_gsmob)
	(gdbscm_mark_eqable_gsmob): Remove these now-unneeded functions.
	Adapt all callers.
	* guile/scm-gsmob.c (gdbscm_mark_gsmob)
	(gdbscm_mark_chained_gsmob, gdbscm_mark_eqable_gsmob): Remove.
	(gdbscm_gsmob_property, gdbscm_set_gsmob_property_x)
	(gdbscm_gsmob_has_property_p, add_property_name)
	(gdbscm_gsmob_properties): Remove, and remove them from gsmob_functions.
	* guile/lib/gdb.scm (gdb-object-property, set-gdb-object-property)
	(gdb-object-has-property?, gdb-object-properties): Remove.
	(gdb-object-kind): Renamed from gsmob-kind.

	doc/
	* guile.texi (GDB Scheme Data Types): Remove documentation for
	removed interfaces.  Update spelling of gdb-object-kind.

	testsuite/
	* gdb.guile/scm-breakpoint.exp:
	* gdb.guile/scm-gsmob.exp: Update to use plain old object
	properties instead of gdb-object-properties.