Reset all files to current develop.
Remove unused usings in NEST project.

Preserve these usings even if they are not used:

* System
* System.Collections.Generic
* System.Linq

Affect:

    src\Nest
    src\Profiling
    src\Serialization
    src\Tests

It doesn't affect src\CodeGeneration and src\Elasticsearch.Net because I'm not sure what code is generated and what isn't.