RFR: 8308342: Remove MetaspaceClosure::Ref::keep_after_pushing()

[Ioi Lam]

More CDS clean up:

- Remove the field `SourceObjInfo::_ref` since we have already cached its contents in fields such as `SourceObjInfo::_msotype` and `SourceObjInfo::_source_addr` 
- Since there's no need to keep the `_ref` around, we can remove the `Ref::keep_after_pushing()` API.
- Also `ArchiveBuilder::clean_up_src_obj_table()` is no longer necessary.

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Commit messages:
 - Fixed bug in last commit
 - more simplication; added assert
 - 8308342: Remove MetaspaceClosure::Ref::keep_after_pushing()

Changes: https://git.openjdk.org/jdk/pull/14042/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14042&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308342
  Stats: 48 lines in 4 files changed: 5 ins; 31 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/14042.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14042/head:pull/14042

PR: https://git.openjdk.org/jdk/pull/14042