RFR: 8278638: Remove FLAG_IS_CMDLINE(UseSharedSpaces)

[Harold Seigel]

Please review this small fix for bug JDK-827638.  The fix was tested by running Mach5 tiers 1 and 2 on Linux, Mac OS, and Windows, Mach5 tier 4 on Linux x64, and by building on linux-aarch64-zero and linux-x64-zero.

Thanks, Harold

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Commit messages:
 - 8278638: Remove FLAG_IS_CMDLINE(UseSharedSpaces)

Changes: https://git.openjdk.java.net/jdk/pull/6835/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6835&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278638
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6835.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6835/head:pull/6835

PR: https://git.openjdk.java.net/jdk/pull/6835