odb/source-packed: drop pointer to "files" parent source

odb/source-packed: drop pointer to "files" parent source

Over the last commits we have turned the packfile store into a proper
object database source that can be used as a standalone backend. As
such, it is no longer necessary to have it coupled to the "files" parent
source.

Remove the pointer to the owning "files" source so that the "packed"
source can be used as a standalone entity.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>