Retire arm_remap_nocache() and the data and constants associated with it.

The only remaining user was the code that allocates bounce pages for armv4
busdma.  It's not clear why bounce pages would need uncached memory, but
if that ever changes, kmem_alloc_attr() would be the way to get it.
Oops, one more instance of ARM_NOCACHE_KVA_SIZE was hiding under the couch.
This should have been cleaned up along with r257201.
Eliminate a compiler warning about extraneous parens.