Remove obsolete code (possible stopper for 9.0)

I found these comments on 9.0.x and maser.
```
$ git grep "remove before 9.0.0 release"
iocore/net/SSLConfig.cc:  // remove before 9.0.0 release
iocore/net/YamlSNIConfig.cc:    // remove before 9.0.0 release
```

The comment says:
```
// remove before 9.0.0 release
// backwards compatibiity
```

Feel free to close this issue if there's already an issue or PR for it.