SUP-2392 - Remove WebVTT uploading support in KMC

Per this change: https://github.com/kaltura/server/commit/32e51df0cae3d64ded79953dc040cba57517fd78, we do not longer support uploading WebVTT captions via the API/server. Hence, when uploading via KMC - Internal Server Error is received. 
So we need to remove the option to upload via the KMC as well.