refactor(memory): use builtin strcat() instead of STRCAT()

The latter was mostly relevant with the past char_u madness.

NOTE: STRCAT also functioned as a counterfeit "NOLINT" for clint
apparently. But NOLINT-ing every usecase is just the same as disabling
the check entirely.

src/nvim/autocmd.c

@@ -711,7 +711,7 @@ char *au_event_disable(char *what)
   if (*what == ',' && *p_ei == NUL) {
     STRCPY(new_ei, what + 1);
   } else {
-    STRCAT(new_ei, what);
+    strcat(new_ei, what);
   }
   set_option_direct(kOptEventignore, CSTR_AS_OPTVAL(new_ei), 0, SID_NONE);
   xfree(new_ei);