Remove cursor enabling and fetch size setting

[dai-chen]

Description

1. Remove old opendistro.sql.cursor.enabled and opendistro.sql.cursor.fetch_size setting
2. [Breaking Change] Change the behavior when fetch_size is absent:
   a. Before changes: open cursor with size read from fetch size setting above
   b. After changes: no cursor context created (same behavior as fetch_size=0)

The reason why to make this breaking change is that cursor support is missing in the new SQL engine. If pagination is expected in the absence of fetch_size, all queries will fallback to old engine (once cursor enabled).

Issues Resolved

#60
opendistro-for-elasticsearch/sql#1081 (comment)

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[penghuo]

Thanks for the change!