RFR: 8267321: Use switch expression for VarHandle$AccessMode lookup

[Claes Redestad]

Using a switch expression instead of a (read-only) static `HashMap` reduces initialization overhead of `VarHandle$AccessMode`. This gets loaded earlier after JDK-8265079, so it started showing up in a few lambda startup tests.

This also obsoletes a jtreg test that only verified that this map was optimally sized.

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Commit messages:
 - Remove obsolete test
 - Transform AccessMode name map into switch expression

Changes: https://git.openjdk.java.net/jdk/pull/4102/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4102&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267321
  Stats: 90 lines in 2 files changed: 31 ins; 56 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4102.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4102/head:pull/4102

PR: https://git.openjdk.java.net/jdk/pull/4102