Remove unused property `MatrixEvent.untrusted` (#5118)
* Remove unused property MatrixEvent.untrusted
This was never set to anything other than `false`. I think it is a hangover
from pre-rust-sdk.
* Remove call to redundant `isKeySourceUntrusted`
`isKeySourceUntrusted` always returns false so no point calling it
* Remove dangling assignments to MatrixEvent.untrusted
