RFR: JDK-8254195: java/nio/file/Files/SubstDrive.java failed with "AssertionError: expected [144951656448] but found [144951640064]"

[Nhat Nguyen]

Remove assertions for FileStore.getUnallocatedSpace() and FileStore.getUsableSpace() since the values are not
guaranteed to be the same on a busy system.

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Commit messages:
 - JDK-8254195: java/nio/file/Files/SubstDrive.java failed with "AssertionError: expected [144951656448] but found [144951640064]"

Changes: https://git.openjdk.java.net/jdk/pull/565/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=565&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254195
  Stats: 8 lines in 1 file changed: 0 ins; 8 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/565.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/565/head:pull/565

PR: https://git.openjdk.java.net/jdk/pull/565