Drop mg2-codegen (Magento 2 Code Generator)

[lmakarov]

There are various minor issues with this tool which make it a PIA to deal with for a docker image maintainer:

• Unable to run without sudo when installed globally staempfli/magento2-code-generator#11
• mg2-codegen installed with Composer does not report its version staempfli/magento2-code-generator#15
• mg2-codegen.phar attached to the 1.11.2 release tag is wrong version staempfli/magento2-code-generator#28

Adding PHP 8.0 support effort for docksal/cli has uncovered those minor issues again.

The tool has to be installed globally via cgr (a safer replacement for composer global install), but cgr has not yet added support for PHP 8.0 (consolidation/cgr#40).

For now, I'd rather keep different PHP image variants consistent in terms of what tools we bundle in docksal/cli.
We can re-introduce mg2-codegen back if there is a user base for it among Docksal users.