code: function removeKey(key, masterPassword) {
      var contents = getContents(masterPassword);
      if (contents[key] != undefined) delete contents[key];
  
      var fileContents = JSON.stringify(contents);
      var encryptedFileContents = encryption.encrypt(fileContents, masterPassword);
      file.writeToFile(encryptedFileContents);
  }
docs: Removes a key from the safetext password file.
  @param key
  @param masterPassword
  @returns {Promise.<TResult>}