外部キー制約の削除が DROP INDEXになる (DROP FOREIGN KEYになって欲しい)
外部キー制約の削除が DROP INDEXになる (DROP FOREIGN KEYになって欲しい)

```
--- FAIL: TestDiff (0.00s)
        Error Trace:    diff_test.go:112
        Error:          Not equal:
                        expected: "ALTER TABLE `fuga` DROP FOREIGN KEY `fk`;\nALTER TABLE `fuga` ADD INDEX `fid` (`fid`);"
                        received: "ALTER TABLE `fuga` DROP INDEX `fk`;\nALTER TABLE `fuga` ADD INDEX `fid` (`fid`);"
        Messages:       result SQL should match
FAIL
FAIL    github.com/schemalex/schemalex/diff     0.011s
```

schemalex/schemalex,soh335/schemalex,schemalex/schemalex