AttributeManager: Fix attribute name during attribute removal
AttributeManager: Fix attribute name during attribute removal

Before this change, the `author` attribute was silently discarded
during `.map()` iteration and the name of the attribute to remove was
included twice with two different values.

storytouch/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite,ether/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite