RFR: 8295012: Arena should not derive from CHeapObj<mtNone>

[Kim Barrett]

Please review this change to the `Arena` class.  Rather than deriving from
`CHeapObj<mtNone>` (which has the effect of preventing C-heap allocation for an
arena by asserting if such is attempted), instead have no allocation base
class so that `new Arena(...)` will fail to compile.

Testing: mach5 tier1

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Commit messages:
 - remove base class from Arena

Changes: https://git.openjdk.org/jdk/pull/10621/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10621&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295012
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10621.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10621/head:pull/10621

PR: https://git.openjdk.org/jdk/pull/10621