get rid of compatibility ThreadStart/ThreadEnd events for virtual threads

The compatible lifecycle ThreadStart/ThreadEnd events were to support legacy virtual thread unaware JVMTI agents which do not enable the can_support_virtual_threads capability. When this capability is enabled then the VirtualThreadStart/VirtualThreadEnd instead of the ThreadStart/ThreadEnd events are generated for virtual threads and can be managed (enabled/disabled) separately. If the the can_support_virtual_threads capability is disabled then the ThreadStart/ThreadEnd events are generated.
This enhancement is to get rid of the compatible lifecycle ThreadStart/ThreadEnd events.
Motivation: Performance overhead from compatible lifecycle events can be significant when a lot of virtual threads are created.

Also, there is an experimental VM flag PostVirtualThreadCompatibleLifecycleEvents (enabled bu default).
If it is turned on then the default compatible lifecycle ThreadStart/ThreadEnd events for virtual threads are generated.
This VM flag has to be removed now.