attrmap: extend -remove to allow removing attributes with any value.

attrmap: extend -remove to allow removing attributes with any value.

Currently, `-remove foo` would only remove an attribute `foo = ""`,
which doesn't work on an attribute like `src` that may have any
value. Extend `-remove` to handle both cases. `-remove foo=""` has
the old behavior, and `-remove foo` will remove the attribute with
whatever value it may have, which is still compatible with the old
behavior.