Remove dead code from the powerpc uma_small_alloc().

[markj]

Summary

32-bit Book-E doesn't set UMA_MD_SMALL_ALLOC, and 32-bit OEA platforms
have a 32-bit vm_paddr_t. Moreover, this code was wrong in that it
leaked the page if the check failed.

[jhibbits]

Yep, been meaning to clean this up since not too long after I initially committed it anyway...