Handle remote type written without turbofish

[dtolnay]

The following already works, almost certainly by accident, but in any case we can't break it:

struct Generic<T> {
    value: T,
}

#[derive(Serialize, Deserialize)]
#[serde(remote = "Generic::<u8>")]
struct Concrete {
    value: u8,
}

However the turbofish in the remote attribute should not be necessary, since conceptually the attribute is being used to refer to a type, not an expression. This PR makes the attribute work as serde(remote = "Generic<u8>").

Related to #2327.