RFR (sh/8): Remove some obsolete Shenandoah code from C2

[Roman Kennke]

This removes a block of code that looks like it's not necessary anymore.
According to hg annotate, it appears to be from our first import into
this tree. It's most likely related to our previous barrier scheme. I
removed the code and run a bunch of tests:

Tests: hotspot_gc_shenandoah (fastdebug/release), specjvm, specjbb

with no ill effects.

Webrev:
http://cr.openjdk.java.net/~rkennke/shjdk8-remove-cruft1/webrev.00/

Can I please get a review?

Thanks,
Roman