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/normal.c

@@ -1987,7 +1987,7 @@ bool add_to_showcmd(int c)
     size_t overflow = old_len + extra_len - limit;
     memmove(showcmd_buf, showcmd_buf + overflow, old_len - overflow + 1);
   }
-  STRCAT(showcmd_buf, p);
+  strcat(showcmd_buf, p);
 
   if (char_avail()) {
     return false;