RFR: JDK-8288573: Make `java.lang.reflect.Executable::getParameterCount()' actually abstract

[Joe Darcy]

Whatever the motivation for how this method was coded when added in JDK 8, since Executable is now a sealed class with Constructor and Method the only allowed subclasses, getParameterCount can be coded as an normal abstract method. (The implementation of getParameterCount in Method and Constructor use fields private to each of those classes so the code cannot easily be shared in Executable).

Please also review the small accompanying CSR: https://bugs.openjdk.org/browse/JDK-8288630

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

Commit messages:
 - JDK-8288573: Make `java.lang.reflect.Executable::getParameterCount()` actually abstract

Changes: https://git.openjdk.org/jdk/pull/9192/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9192&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288573
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9192.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9192/head:pull/9192

PR: https://git.openjdk.org/jdk/pull/9192