Remove error message causing segfault

This was utterly stupid. The check right above ensured that `sym` was NULL,
ergo that the argument to `yyerror` *would* segfault.

The only two call sites cannot pass a non-NULL pointer anyways, which I'm
betting is why this went unnoticed.
I did what an optimizing compiler would do anyways: remove the dead code.

src/asm/symbol.c

@@ -249,19 +249,6 @@ void sym_Purge(char const *tzName)
 	}
 }
 
-/*
- * Get a string equate's value
- */
-char *sym_GetStringValue(struct sSymbol const *sym)
-{
-	if (sym != NULL)
-		return sym->pMacro;
-
-	yyerror("String symbol '%s' not defined", sym->tzName);
-
-	return NULL;
-}
-
 /*
  * Return a constant symbols value
  */