RFR: 8310999: Add @since info in jdk.jsobject files

[Roger Riggs]

Source code cleanup.
Add @since, remove unused imports and remove unnecessary SuppressWarnings of deprecation.

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Commit messages:
 - 8310999: Add @since info in jdk.jsobject files

Changes: https://git.openjdk.org/jdk/pull/14775/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14775&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310999
  Stats: 9 lines in 2 files changed: 1 ins; 5 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/14775.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14775/head:pull/14775

PR: https://git.openjdk.org/jdk/pull/14775