RFR: 8338785: The java.awt.datatransfer.SystemFlavorMap#FLAVOR_MAP_KEY field is not used

[Sergey Bylokhov]

The unused `java.awt.datatransfer.SystemFlavorMap#FLAVOR_MAP_KEY` field is deleted.

It was unused since [8037485](https://bugs.openjdk.org/browse/JDK-8037485): Refactor java.awt.datatransfer to eliminate dependency on AWT.

See:
https://github.com/openjdk/jdk/commit/0fecd7769453dfb3ef1609b057a9e69aee57d0c1#diff-2c0c7e0dbcabe2bced2907f383bdb4e2a3ed64686c219f37f7c6469ecc47d4baL198

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Commit messages:
 - 8338785: The java.awt.datatransfer.SystemFlavorMap#FLAVOR_MAP_KEY field is not used

Changes: https://git.openjdk.org/jdk/pull/20670/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20670&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338785
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20670.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20670/head:pull/20670

PR: https://git.openjdk.org/jdk/pull/20670