Make Executable.getParameterCount() actually abstract

Summary

Change Executable.getParameterCount to be declared to be an abstract method, matching its semantics.

Problem

While Executable.getParameterCount is conceptually an abstract method, it is not coded as such, which is confusing. Since Executable is now sealed, only the implementation in Mehtod and Constructor are relevant.

Solution

Change the method to be declared as abstract.

Specification

--- a/src/java.base/share/classes/java/lang/reflect/Executable.java
+++ b/src/java.base/share/classes/java/lang/reflect/Executable.java
@@ -253,9 +253,7 @@ public abstract sealed class Executable extends AccessibleObject
      * @return The number of formal parameters for the executable this
      * object represents
      */
-    public int getParameterCount() {
-        throw new AbstractMethodError();
-    }
+    public abstract int getParameterCount();

     /**