Eliminate the record extension operator from the Value type returned by the partial evaluator

It was used only in cases where a lock field needed to be added to a
run-time variable, like e.g. in examples/phrasebook/SentencesTha.gf: 

    lin
      PGreetingMale g   = mkText (lin Text g) (lin Text (ss "ครับ")) | g ;
      PGreetingFemale g = mkText (lin Text g) (lin Text (ss "ค่ะ")) | g ;

But lock fields are only meaningful during type checking and can safely be
ignored in later passes.

src/compiler/GF/Compile/Compute/Value.hs

@@ -33,7 +33,7 @@ data Value
   | VAlts Value [(Value, Value)]
   | VStrs [Value]
 -- -- | VGlue Value Value -- hmm
-  | VExtR Value Value -- hmm
+-- --  | VExtR Value Value -- hmm
   | VError String
   deriving (Eq,Show)