RFR: 8341776: Remove unused enum values from LambdaForm$Kind

[Claes Redestad]

Trivially removing some unused left-over `LambdaForm$Kind` enums. 

- Last use of `COLLECT` and `SPREAD` was removed by fac39fe9
- `FIELD` was added by 8d5f5b9a but had no usage from the start
- Last use of `CONVERT` was removed by eda5f090

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Commit messages:
 - 8341776: Remove unused enum values from LambdaForm

Changes: https://git.openjdk.org/jdk/pull/21592/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21592&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341776
  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21592.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21592/head:pull/21592

PR: https://git.openjdk.org/jdk/pull/21592