Also remove the JDWP supportsVirtualThreads capability. These were added as a temporary measure so debuggers can determine if the target JDK supports virtual thread. It has been needed because the "current" JDK could be based either on the loom repo or jdk mainline repo, both with the same version number, so the version number could not be used to determine if virtual threads was supported.

Once loom is integrated into jdk mainline, debuggers can use the JDK version number to determine if virtual threads are supported, and supportsVirtualThreads() will no longer be needed. However, it can't be removed at the time of integration, because this won't give debuggers a chance to transition off of its use, but it should be removed before RDP1 of the release that loom is integrated into.

The following will need to be removed:

In jdwp.spec:

              (boolean supportsVirtualThreads
                       "Does the VM support virtual threads, temporary capability")

In the JDI VirtualMachine.java:

     default boolean supportsVirtualThreads() {
         return false;
     }

In the debug agent VirtualMachineImpl.c:

    /* 22 supportsVirtualThreads, temporary capability */
    (void)outStream_writeBoolean(out, (jboolean)JNI_TRUE);