[TableGen] Remove unused tokens FalseKW and TrueKW

These were introduced in https://reviews.llvm.org/D90635 but never used.

Author: jayfoad

llvm/lib/TableGen/TGLexer.h

@@ -80,15 +80,13 @@ enum TokKind {
   Code,
   Dag,
   ElseKW,
-  FalseKW,
   Field,
   In,
   Include,
   Int,
   List,
   String,
   Then,
-  TrueKW,
 
   // Object start tokens.
   OBJECT_START_FIRST,