Remove the manual deserialization of `Activity` (#1629)
Remove the manual deserialization of `Activity` (#1629)

The serde attribute `#[serde(deserialize_with = "..")]` is now used to handle
the deserialization of the buttons and `#[serde(default)]` is sufficient
for types implementing the `Default` trait.
acdenisSK/serenity,Flat/serenity