Remove check_{attach,detach} from volumes API

These methods are not called from anywhere and are likely just a relic
from splitting Cinder out from Nova. Moreover their usage from the API
should really be discouraged as they are inherently racy (the result is
immediately outdated since it is not atomic with anything that might
follow).

Partial-bug: #1302774
Change-Id: I5d79b22b2a1d3c9be429a13d094328fc2075fcd2
Merge "Remove check_{attach,detach} from volumes API"