[16] RFR: 8241065: Shenandoah: remove leftover code after JDK-8231086

[Charlie Gracie]

Hi,

Please review this change to remove the unused ArrayCopyStoreValMode enum. 
The Bug also mentions removing the write_ref_array_pre_work function but it was
previously removed as part of JDK-8242130 [1].

Bug: https://bugs.openjdk.java.net/browse/JDK-8241065
Webrev: http://cr.openjdk.java.net/~burban/cgracie/JDK-8241065/webrev0.0/

Sponsor required: Yes

Test:
   Built on MacOS {release,fastdebug}

Thanks,
Charlie Gracie

[1] https://bugs.openjdk.java.net/browse/JDK-8242130