vttablet shouldn't require its own mysql client binary

[derekperkins]

For at least ApplySchema and potentially other commands, vttablet looks to use a local binary instead of using the same one in the mysql container. We should remove that dependency.

// execute the schema change using an external mysql process
// (to benefit from the extra commands in mysql cli)

https://github.com/youtube/vitess/blob/master/go/vt/mysqlctl/schema.go#L341