Fix drush setup

[acbramley]

I've been trying to debug some issues I'm having with running cron on the platform. I have a custom bin-dir to install composer binaries in /app/bin/ instead of /app/vendor/bin because that is what we're used to. However, this seems incompatible with the platform.

Firstly #376 setup a hardcoded alias to /app/vendor/bin/drush but there are a lot more problems:

There's a global drush script installed in /usr/local/bin/drush which is an ancient version (8.4.12)

[canceraus3]master@cli-drupal:/app$ which drush
/usr/local/bin/drush

[canceraus3]master@cli-drupal:/app$ /usr/local/bin/drush --version
Drush Launcher Version: 0.10.2
 Drush Version   :  8.4.12 

This seems to be being used at some points (ignoring the alias).

But the alias is used when running just drush

[canceraus3]master@cli-drupal:/app$ drush --version
bash: /app/vendor/bin/drush: No such file or directory

This seems to be causing issues with the custom cron job wrapper https://www.govcms.support/support/tickets/26269

It should be possible to set a custom bin-dir on this platform.

We should also remove the /usr/local/bin/drush file.