Remove dead code related to prioritized buildpacks

[#68432850]

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>
Need to call destroy to exercise after_destroy

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>
Extract BuildpackPositioner

An attempt to reduce the logic in Buildpack and eventually write clearer specs around the positioning

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>
Encapsulate shift_to_position in update

- update was the only method using shift_to_position
- there was a second (sorta nested) transaction that was not needed
- [#68432850]

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>
Reduce knowledge about the db in positioner

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>