RFR: Remove _keepalive WeakHandle remnants.

[Coleen Phillimore]

Since I went down the path of looking at _keepalive and found that the code for it was removed, I think the rest of it should be also.
Tested with loom-tier1, 2.

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Commit messages:
 - Remove Continuation::weak_storage
 - Remove _keepalive WeakHandle remnants.

Changes: https://git.openjdk.java.net/loom/pull/41/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=41&range=00
  Stats: 81 lines in 15 files changed: 0 ins; 47 del; 34 mod
  Patch: https://git.openjdk.java.net/loom/pull/41.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/41/head:pull/41

PR: https://git.openjdk.java.net/loom/pull/41