Move cipher_null check in internal function crypt_is_cipher_null.

Also removes tools helper so that we keep check in one place.

src/utils_tools.c

@@ -435,14 +435,6 @@ int tools_wipe_progress(uint64_t size, uint64_t offset, void *usrptr)
 	return r;
 }
 
-int tools_is_cipher_null(const char *cipher)
-{
-	if (!cipher)
-		return 0;
-
-	return !strcmp(cipher, "cipher_null") ? 1 : 0;
-}
-
 /*
  * Keyfile - is standard input treated as a binary file (no EOL handling).
  */