Clarify leftover FIXMEs in client's code gen services

[ndoschek]

Investigate // FIXME comments and if marked code needs to be replaced or can be removed:

• https://github.com/eclipse-emfcloud/coffee-editor/blob/master/web/coffee-cpp-extension/src/browser/generate-code-service.ts
• https://github.com/eclipse-emfcloud/coffee-editor/blob/master/web/coffee-java-extension/src/browser/generate-code-service.ts