[Java] Remove public static helper method in vector classes for JSONReader/Writer

[asfimport]

Reporter: Li Jin / @icexelloss
Assignee: Li Jin / @icexelloss

PRs and other links:

• GitHub Pull Request #1290

_{Note: This issue was originally created as ARROW-1717. Please see the migration documentation for further details.}

[asfimport]

Siddharth Teotia / @siddharthteotia:
Resolved by PR #1290