Change the bitmap callback signature to have a void return value.

Change the bitmap callback signature to have a void return value.

Previously, a bitmap callback could abort the bitmap traversal by
returning false.  All of the bitmap callbacks used in the garbage
collector are hardwired to return true.  This change eliminates the
early termination facility and its attendant complexity.

Change-Id: I5490dd74b56b9700ec1b7e657637fae5eb4cc339