RFR: 8256813: Simplify WeakProcessor counting of OopStorage entries

[Kim Barrett]

Please review this change to WeakProcessor's counting of weak and dead
entries.  After JDK-8212879 we no longer need different closure classes to
deal with the serial and oopstorage cases, since the serial case no longer
exists.  We can collapse that down to one closure class that is tailored for
the remaining uses.

Also deleted a stale comment left over from JDK-8212879.

Testing:
mach5 tier1
Locally (linux-x64) hotspot:tier1 with Serial and Parallel GCs.

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Commit messages:
 - simplify

Changes: https://git.openjdk.java.net/jdk/pull/1362/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1362&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256813
  Stats: 52 lines in 3 files changed: 9 ins; 27 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1362.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1362/head:pull/1362

PR: https://git.openjdk.java.net/jdk/pull/1362