Remove unused empty production for `litsq_zq` in texvccheck

It is impossible to take the empty alternative for this production,
since any input which matches `litsq_aq` would instead match `lit_aq`
if the empty alternative is used.  In addition, there is no action
clause for the empty alternative, so it wouldn't typecheck if the
empty alternative was possible.

This has no effect on the generated parser, but clarifies the grammar
(and makes ports easier!).

Change-Id: I32c85d2c74ab047b4b52401248d5dc7fc3201705

texvccheck/parser.mly

@@ -44,7 +44,7 @@ litsq_uq:
 litsq_dq:
     litsq_zq SUB lit		{ $1,$3 }
 litsq_zq:
-  | SQ_CLOSE 			{ TEX_LITERAL sq_close_ri }
+    SQ_CLOSE			{ TEX_LITERAL sq_close_ri }
 expr_nosqc:
     /* */			{ [] }
   | lit_aq expr_nosqc		{ $1 :: $2 }