Remove backwards compat hack from EventContext serialzation code

[H-Shay]

#15233 changed the format of EventContext and as such required two small hacks to make the serialization backwards compatible:

synapse/synapse/events/snapshot.py

Lines 206 to 212 in 553f2f5

	# workaround for backwards/forwards compatibility: if the input doesn't have a value
	# for "state_group_deltas" just assign an empty dict
	state_group_deltas = input.get("state_group_deltas", None)
	if state_group_deltas:
	state_group_deltas = _decode_state_group_delta(state_group_deltas)
	else:
	state_group_deltas = {}

synapse/synapse/events/snapshot.py

Lines 189 to 191 in 553f2f5

	# add dummy delta_ids and prev_group for backwards compatibility
	"delta_ids": None,
	"prev_group": None,

We should remove these after a few (let's say 4?) releases. The second one (lines 189-191) can just be removed, and the first one just requires a change:

state_group_deltas = _decode_state_group_delta(input["state_group_deltas"])

and the removal of the if statement.