[FLINK-10528] [table] Remove methods that were deprecated in Flink 1.4.0

[fhueske]

What is the purpose of the change

• Remove Table API methods that were deprecated in Flink 1.4.0.

Brief change log

• remove TableEnvironment.sql() deprecated with FLINK-6442
• remove StreamTableEnvironment.toDataStream() and TableConversions.toDataStream() deprecated with FLINK-6543
• remove Table.limit() deprecated with FLINK-7821

Verifying this change

• Run existing tests

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

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? I checked that the removed methods are not referenced in the docs or code comments.

[twalthr]

Looks good to me. +1 from my side. Could you also perform a quick search if these methods are used in the website docs as well?

[hequn8128]

+1 to merge.

I searched these methods in the Main source repository and find no occurrence.
https://github.com/apache/flink.git

There are some occurrences in the Website repository. However it is not related to this PR.
https://github.com/apache/flink-web.git

The occurrences are:
sql()
- 2016-05-24-stream-sql.md
- 2017-03-30-dynamic-tables.md
toDataStream()
- 2017-03-29-table-sql-api-update.md
limit()
- None

[fhueske]

Thanks for the reviews.
I think we can leave the old blog posts as they are.

[fhueske]

merging