8382257: Shenandoah: Clean up stack watermark barrier support

[shipilev]

Following the nmethod barrier code cleanup, this also cleans up stack watermark machinery, which is not unconditionally on. There is a little conflict in tests due to the absence of JDK-8369013, which has some bugtail in itself. So on balance, it is easier to accept the conflict here.

Additional testing:

• Linux x86_64 server fastdebug, hotspot_gc_shenandoah

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Issue

• JDK-8382257: Shenandoah: Clean up stack watermark barrier support (Enhancement - P4)

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