PHP API ContentService::removeTranslation, which removes Translation from all the Versions of a given Content, doesn't cleanup Field Type External Storage when performing this operation.

Steps to reproduce:
1. Create Content which has Field Types using Ext. Storage (e.g. ezkeyword)
2. Translate it into another language, use different words to distinguish translation.
3. Using ezplatform:remove-content-translation command (introduced some time ago, probably still in master) remove previously created Translation from the created Content.
4. Observe that in database tables used by External Storage of FT (e.g. ezkeyword, ezkeyword_attribute_link) there are references to deleted fields (attributes).