RFR: 8370878: C1: Clean up unnecessary ConversionStub constructor

[Zihao Lin]

C1: Clean up unnecessary ConversionStub constructor
Remove class which should not reach.

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Commit messages:
 - C1: Clean up unnecessary ConversionStub constructor

Changes: https://git.openjdk.org/jdk/pull/28096/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28096&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370878
  Stats: 42 lines in 3 files changed: 0 ins; 39 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/28096.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28096/head:pull/28096

PR: https://git.openjdk.org/jdk/pull/28096