[memcached] Removed groupId (duplicates parent).

For reference, other bindings don't appear to specify a groupId at this level
in their pom.xml files.

@shivam-maharshi says in a [comment][1]:

The groupId here is the duplicate of the parent groupId. This is creating a
problem in the maven build.

[1]: https://github.com/brianfrankcooper/YCSB/commit/d4b1d247a31cf9fbf42b2e324b5b1cc27fdd9565#commitcomment-15030853