Remove DRUSH_PATHS_CACHE_DIRECTORY - Drush 11+ doesn't have its own cache anymore

[bkosborne]

The post-site-install.sh and db-update.sh factory hooks provided here include some code to get a unique cache directory to use for drush and to set it via a drush environment var DRUSH_PATHS_CACHE_DIRECTORY. Drush 11 (which BLT 13.x requires) removed its own cache, so this code is not needed anymore.

See:

• https://github.com/drush-ops/drush/releases/tag/11.0.0
• Remove Drush's cache API. We no longer need to cache annotation parsing. drush-ops/drush#4965
• More removal of cache dir drush-ops/drush#4971

[dan612]

I believe this was resolved via dbaafb6