8382150: Shenandoah: Clean up nmethod entry barrier support

[shipilev]

We have this transitional method, ShenandoahCodeRoots::use_nmethod_barriers_for_mark(), added back when nmethod entry barriers were a new features. But today, continuations are always on, and class unloading is also always on. So there is no point it running in the mode where nmethod entry barriers are disabled. This is a clean up that is useful in further optimizations past Late Barrier Expansion.

Additionally, ShenandoahNMethod serves as the wrapper over *NMethodBarrierSet calls, so I cleaned up part of that interface as well.

Additional testing:

• Linux x86_64 server fastdebug, hotspot_gc_shenandoah

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Issue

• JDK-8382150: Shenandoah: Clean up nmethod entry barrier support (Enhancement - P4)

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[mlbridge]

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• 00: Full (7919fd68)

[earthling-amzn]

Is there diagnostic value in having the capability to not use nmethod barriers? Would we ever turn them off for passive mode?