RocksDB: remove default write_buffer_size of 64MB
RocksDB: remove default write_buffer_size of 64MB

Now we are configuring the write buffer size for default & seqno
column families via OptimizeLevelStyleCompaction(), we shouldn't set a
default for it in rocksdb_cf_options - as that would override the
value calculated in OptimizeLevelStyleCompaction().

Change-Id: Id2b0a1acf6a21a04c3ca11df0939bf376a2edd12
Reviewed-on: http://review.couchbase.org/85584
Reviewed-by: Paolo Cocchi <9dd88c9f3e5cbab6e19a6c020f107e0c648ac956@couchbase.com>
Tested-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine