Remove PartialMerge implementation from Cassandra merge operator

[sagar0]

PartialMergeMulti implementation is enough for Cassandra, and PartialMerge is not required. Implementing both will just duplicate the code.
As per https://github.com/facebook/rocksdb/blob/master/include/rocksdb/merge_operator.h#L130-L135 :

  // The default implementation of PartialMergeMulti will use this function
  // as a helper, for backward compatibility.  Any successor class of
  // MergeOperator should either implement PartialMerge or PartialMergeMulti,
  // although implementing PartialMergeMulti is suggested as it is in general
  // more effective to merge multiple operands at a time instead of two
  // operands at a time.

Test Plan:
make check -- Run all unit tests.

[sagar0]

@scv119, @wpc -- Can you take a look? (Seems like I can't add you as reviewers).

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[scv119]

Good catch! Thanks

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