RFR: 8264997: Remove SystemDictionary::cache_get

[Coleen Phillimore]

This is a trivial change to remove an unneeded function in SystemDictionary.  The pd_cache_table() that is owned by SystemDictionary is already available to callers of SystemDictionary, ie. the ServiceThread, so there's no need to hide access inside of a function with a backwards name.

I also fixed add_protection_domain logging to be useful and fixed an assert.

Tested with mach5 tier1-3 in progress and manually.

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Commit messages:
 - 8264997: Remove SystemDictionary::cache_get

Changes: https://git.openjdk.java.net/jdk/pull/3418/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3418&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264997
  Stats: 19 lines in 4 files changed: 7 ins; 7 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3418.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3418/head:pull/3418

PR: https://git.openjdk.java.net/jdk/pull/3418