Add the method removeTranslation WIP

Hello :)

I need the removeTranslation method to use in another entity and link properly with all usuals bundles. 

This permit to use the lexik translations with created entities and link them with the normal translation process easily. Another usefull case is to display specialized forms for filtered keys registered in project entities.

I can write other code, or correct dependances if needed (tests particularly).

Hoping, this can be merge in stable branche.

Best regard, and many thanks for this usefull bundle
Merge pull request #57 from nicodmf/patch-1

Add the method removeTranslation()