RFR: JDK-8275440: Remove VirtualSpaceList::is_full()

[Thomas Stuefe]

This function is only used from a single gtest. Ioi remarked that it is not threadsafe. The context in which it is used is not multithreaded, but nevertheless I opted to remove the function and modified the test to make the condition somewhat more generic.

This trades a slight decrease in complexity with a very minor decrease in test precision, but that is fine.

Test: 
- manual gtests
- GHAs
- ran the metaspace jtreg gtests, which execute gtests with various metaspace modes

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Commit messages:
 - remove function

Changes: https://git.openjdk.java.net/jdk/pull/6147/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6147&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275440
  Stats: 15 lines in 3 files changed: 0 ins; 13 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6147.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6147/head:pull/6147

PR: https://git.openjdk.java.net/jdk/pull/6147