Remove member bitmaps when linking to objectpool

Closes #1728 (closed).

After a repository joins an object pool there can be an intermediate state where both the new member repository and the pool repository have a packfile bitmap. Git expects there to be at most one bitmap amongst all its packfiles. This change ensures that if both the pool and the member have a bitmap, the member bitmap gets removed.

This will not cause data loss because packfile bitmaps are optional: Git can function without them. They just make Git faster.