Sliding Sync proxy pollers might benefit from calling oldschool /sync with a filter that omits presence

[reivilibre]

I noticed that /sync lets you apply filters to presence (https://spec.matrix.org/v1.7/client-server-api/#filtering).

Given that the SS proxy does not use presence currently, it seems to me like it may be beneficial to request the homeserver not to include any presence information.
This would hopefully reduce the frequency of /sync wakeups, reducing load on both the homeserver and the SS proxy pollers.

This might help a little bit with the high CPU usage reported at matrix-org/synapse#15980.

(Related: don't set presence status in SS proxy pollers: #217)