In the 3.0 docs, it says:

All remove() operations for a sharded collection that specify the justOne option must include the shard key or the _id field in the query specification. remove() operations specifying justOne in a sharded collection without the shard key or the _id field return an error.

From 3.2 onwards, the "justOne" qualifier is omitted:

All remove() operations for a sharded collection must include the shard key or the _id field in the query specification. remove() operations without the shard key or the _id field return an error.

However, versions 3.2+ work the same as 3.0 – a shard key/_id is required only if justOne is true.