Remove stale "register" bits from gdb.cp/misc.cc

Remove stale "register" bits from gdb.cp/misc.cc

gdb.cp/misc.cc seems to have been originally copied from
gdb.cp/classes.cc.  The testcases that use it, misc.exp and
inherit.exp don't reference the "register" bits anywhere.  Remove
them, since they trigger warnings with newer GCCs, given "register" is
being removed in C++17.

gdb/testsuite/ChangeLog:

	* gdb.cp/inherit.exp: No longer pass -Wno-deprecated-register.
	* gdb.cp/misc.exp: No longer pass -Wno-deprecated-register.
	* gdb.cp/misc.cc (class small, small::method, marker_reg1)
	(register_class): Delete.
	(main): Don't call register_class.