Remove special case for css in OC.filePath

Fixes the returned path from `OC.filePath` when trying to get the path to a css file, i.e.

`OC.filePath('files','css','foo.css')`

before:
`"/owncloud/index.php/apps/files/css/foo.css"`
after:
`"/owncloud/apps/files/css/foo.css"`

The special case was previously needed when css files were served with php

Note that this bug breaks apps who are using `OC.addScript`

cc @DeepDiver1975 @PVince81