Avoid ReparseDocument on ActiveDocumentChanged. This is already handled ...

...by the Language service.

This had already been removed: https://github.com/fsharp/fsharpbinding/commit/e1dfc52fe629684e1a3a91fc0a8c6bd7f70ec212
but probably reappeared by mistake in 
https://github.com/fsharp/fsharpbinding/commit/a3c755bd8d1a7420f2c43fbe1151f3ed5d002f67