message_send: Remove TypeGuard.
message_send: Remove TypeGuard.

The type safety of a TypeGuard is unchecked by mypy.  While this
particular TypeGuard is safe given the current context, one could
imagine future changes that make it unsafe, so it’s preferable to
avoid unchecked constructs whenever possible.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip