Drop document generation commands

[alcaeus]

See #479 (comment): DocumentGenerator has been dropped from ODM, so the respective commands need to be dropped from the bundle in time for Beta 1.

[cmukanisa]

which command will I use to generate documents for my database because ** #php bin/console doctrine:generate:mongodb:document** don't work

[jmikola]

@christiankasse: See #498 (comment). Please keep the ongoing discussion to that issue.

[cmukanisa]

I apologize but I can not work with ODM because I have left a way out that I have been told to work on, can you help me by explaining what to do? now because I'm totally stuck, please