Remove `RemoveEventStreamOperations` model transformer from `ServerCodegenVisitor` (#1764)

This is a holdover from when the server subproject was started. We've
never utilized this model transformer, nor will we have any use for it
now, since event stream operations are supported in the server since #1479.

See https://github.com/awslabs/smithy-rs/pull/1762#discussion_r978151466.