RFR: 8294608: Remove redundant unchecked suppression in FileDescriptor

[Andrey Turbanov]

Found this redundant suppressions by IntelliJ IDEA inspection.
It seems [JDK-8189330](https://bugs.openjdk.org/browse/JDK-8189330) made them redundant.

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Commit messages:
 - [PATCH] Remove redundant unchecked suppression in FileDescriptor

Changes: https://git.openjdk.org/jdk/pull/10473/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10473&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294608
  Stats: 5 lines in 1 file changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10473.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10473/head:pull/10473

PR: https://git.openjdk.org/jdk/pull/10473