RFR(XXS) 8214029 Remove dead code BasicHashtable::bulk_free_entries

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8214029
http://cr.openjdk.java.net/~iklam/jdk12/8214029-dead-code-hashtable-bulk-free.v01/

This function, and the associated class 
BasicHashtable::BucketUnlinkContext are no
longer used. The code should be removed to prevent bit rot.

Thanks
- Ioi