[hotfix][filesystem] Remove incorrect equals methods in StreamWriters

[pnowojski]

This pull request removes incorrect equals methods in StreamWriterBase (and in classes that inherit from it) that were used for tests and moves their logic to test class.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[tillrohrmann]

LGTM. +1 for merging.

[pnowojski]

Thanks, fixed travis failure and merged.