c++: simplify TEMPLATE_TEMPLATE_PARM hashing

c++: simplify TEMPLATE_TEMPLATE_PARM hashing

r10-7815-gaa576f2a860c82 added special hashing for TEMPLATE_TEMPLATE_PARM
to work around non-lowered ttps having TYPE_CANONICAL set but lowered
ttps did not.  But ever since r13-737-gd0ef9e06197d14 this is no longer
the case, and all ttps should now have TYPE_CANONICAL set.  So this
special hashing is now unnecessary and we can fall back to always using
TYPE_CANONICAL.

gcc/cp/ChangeLog:

	* pt.cc (iterative_hash_template_arg): Don't hash
	TEMPLATE_TEMPLATE_PARM specially.