cbbackupmgr vbucket distribution may cause a panic when using a vbucket-filter

What's the issue?
When using a vbucket-filter it's possible to back the vbucket-distribution code into a position where it attempts access an index which is out of bounds.

What's the fix?
We no longer see a huge benefit from using a vbucket-distribution because we always have to request the streams at the beginning of the backup, therefore, it can simply be removed.