Remove unneeded 'async' dependency

Co-Authored-By: Jakob Ackermann <das7pad@outlook.com>

libraries/access-token-encryptor/lib/coffee/AccessTokenEncryptor.coffee

@@ -1,5 +1,4 @@
 crypto = require('crypto')
-async = require('async')
 
 ALGORITHM = 'aes-256-ctr'