Don't check logger expression for validity.

Essentially we just want to allow everything that is syntactically
correct Erlang.

Change-Id: Ie120c79d0155ff0a2ec927a6d8081b5bae0d48b5
Reviewed-on: http://review.couchbase.org/37519
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>