RFR: 8373984: Check for macos 11 in CGraphicsDevice.m can be removed

[Matthias Baesken]

After we switched to minimum version macOS 11 with [JDK-8317970](https://bugs.openjdk.org/browse/JDK-8317970) a while ago, the check in java.desktop/macosx/native/libawt_lwawt/awt/CGraphicsDevice.m can be removed.

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Commit messages:
 - JDK-8373984

Changes: https://git.openjdk.org/jdk/pull/28901/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28901&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373984
  Stats: 13 lines in 1 file changed: 4 ins; 6 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/28901.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28901/head:pull/28901

PR: https://git.openjdk.org/jdk/pull/28901