Revert "ReplicatedPG: block scrub on blocked object contexts"

This reverts commit e66f2e36c06ca00c1147f922d3513f56b122a5c0.
Reviewed-by: Sage Weil <sage@inktank.com>

0f3235d46c8fd6c537bd4aa8a3faec6c00f311a8 is the firefly commit
corresponding to e66f2e36c06ca00c1147f922d3513f56b122a5c0.

(cherry picked from commit 84728058dbb91b8ed062240b3373b18078f0c9ca)