Remove Drush support

The DrupalApcCache class assumes that any request to clear the cache done from the command line is done from Drush, and the request needs to be executed via the HTTP server, but that is not always the case. For example, when tests run from the command line, the request to clear the cache should be immediately handled. There could be any other cases where sending the request to the HTTP server instead of handling it causes issues.

The correct way to to remove APCu data for the server is implementing a new Drush command, for example drush apc-clear, which could also be generally used to clear any data stored on APCu.
For the moment, I am going to remove the Drush support.