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/fold.c

@@ -3315,7 +3315,7 @@ void f_foldtext(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     char *r = xmalloc(len);
     snprintf(r, len, txt, dashes, count);
     len = strlen(r);
-    STRCAT(r, s);
+    strcat(r, s);
     // remove 'foldmarker' and 'commentstring'
     foldtext_cleanup(r + len);
     rettv->vval.v_string = r;