Revert "ML2 plugin should not delete ports on subnet deletion"

This reverts commit 0d131ff0e9964cb6a65f64809270f9d597c2d5d1

There is really no problem with this change. However, it is probably
triggering a port_update notification to the agent for each port
with an allocated IP.
The agent handles that notification in a way which might be improved
from a scalability perspective.

I don't actually want this change to removed, I am just checking
whether neutron without it passess jobs.

Change-Id: I5494b607127b261043dcddfdc10c93a28ec20af5
Related-Bug: 1253896
Related-Bug: 1254236
Merge "Revert "ML2 plugin should not delete ports on subnet deletion""