RFR: 8357178: Simplify Class::componentType

[Chen Liang]

`isArray` and null return is now redundant when `componentType` is changed to an explicit field.

-------------

Commit messages:
 - 8357178: Simplify Class::componentType

Changes: https://git.openjdk.org/jdk/pull/25280/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25280&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357178
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25280.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25280/head:pull/25280

PR: https://git.openjdk.org/jdk/pull/25280