Respect parent path decision when cleaning paths.

When cleaning the video library and a source containing TV shows was unavailable,
choosing to keep it only kept the source root, while subdirectories were
removed.
The result was that TV shows "disappeared".

This is fixed by checking if there's a decision for the parent path when
cleaning the path table.

See: https://github.com/xbmc/xbmc/pull/5324#issuecomment-54574275