Remove useless assignments in add_to_list functions.

The code was probably pulled from some of the header processing
functions, which were in a loop.  In those versions, "last" pointed to
a collection of new entries appended, but in these two functions,
updating last just obfuscates the code.

mutt/regex.c

@@ -175,12 +175,9 @@ int mutt_regexlist_add(struct RegexList **rl, const char *str, int flags, struct
     t = mutt_regexlist_new();
     t->regex = rx;
     if (last)
-    {
       last->next = t;
-      last = last->next;
-    }
     else
-      *rl = last = t;
+      *rl = t;
   }
   else /* duplicate */
     mutt_regex_free(&rx);