libstdc++: Revert accidentally committed change to bits/stl_iterator.h

In commit r14-3134-g9cb2a7c8d54b1f I only meant to change some uses of
__clamp_iter_cat to use __iter_category_t, I didn't mean to commit the
additional change introducing __clamped_iter_cat_t. This reverts that
part.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (__clamped_iter_cat_t): Remove.