Remove unused empty production for litsq_zq in texvc

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.
This commit matches a25078ff9d to
texvccheck.

Change-Id: I52eddd7321bd8450b47a5d57f2d704f0dd669f16

math/parser.mly

@@ -45,7 +45,7 @@ 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 }