patch 8.1.0745: compiler warnings for signed/unsigned string

Problem:    Compiler warnings for signed/unsigned string.
Solution:   Remove type casts. (John Marriott)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 112b67d..6f9f1b9 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -2290,7 +2290,7 @@
 	/* check these explicitly for a more specific error message */
 	if (*ea.arg == '*' || *ea.arg == '+')
 	{
-	    errormsg = (char_u *)_(e_invalidreg);
+	    errormsg = _(e_invalidreg);
 	    goto doend;
 	}
 #endif