Remove EventFileValidation (digital-asset#1579)

We never actually emit this event so it’s pretty much useless. If we
do want to add progress reporting at some point, we should go with the
recently added official support for that in LSP
microsoft/language-server-protocol#70 (comment).

src/Development/IDE/Types/LSP.hs

@@ -49,7 +49,4 @@ data Event
       -- resource @resource@ changed to @contents
       -- NOTE(JM,MH): Keep the contents lazy as we rely on it in
       -- 'manageOpenVRs'.
-    | EventFileValidation Int Int
-      -- ^ @EventFileValidation finishedValidations totalValidations @
-      -- How many validations have we finished of how many total.
       deriving Show