kern/rman: remove rman_reserve_resource_bound(), partially revert 13fb6657723

Not once has rman_reserve_resource_bound() ever been used. There are
though several uses of RF_ALIGNMENT. In light of this remove this
extra and leave the actually used portion in place.

This partially reverts commit 13fb6657723c1e9cb47bbd286942b432a4306b96.

Reviewed by: imp,jhb
Pull Request: https://github.com/freebsd/freebsd-src/pull/1224