MB-20940: [38/n] Tap Removal - Remove MEMCACHED_REQS_TAP_EVENT from usage
MB-20940: [38/n] Tap Removal - Remove MEMCACHED_REQS_TAP_EVENT from usage

The MEMCACHED_REQS_TAP_EVENT environment variable is no longer used.
Therefore remove its description from the usage printout.

Change-Id: I6ab1fd0169178090a7c5f384d60f62f6eaae92e8
Reviewed-on: http://review.couchbase.org/82156
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

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