RFR: [10+9] Temporarily disable String deduplication

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/strdedup-disable/webrev.01/

We have identified some problems with experimental implementation of String Dedup. Let's disable it
until we fix it, hopefully soon. This changes makes tests reliably pass, and protects users from
using potentially broken feature. I would push the variant of this fix to sh/jdk9 too.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey