RFR: 8289431: Avoid redundant HashMap.get in ZipFileSystemProvider.removeFileSystem

[Andrey Turbanov]

There is overload method HashMap.remove(key,value) which also checks value equality.
It's shorter and faster than pair get+remove.

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Commit messages:
 - [PATCH] Remove redundant HashMap.get call before HashMap.remove

Changes: https://git.openjdk.org/jdk/pull/9314/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9314&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289431
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9314.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9314/head:pull/9314

PR: https://git.openjdk.org/jdk/pull/9314