Since is_visible was refactored to use a variable, these mocks
are not required.
Also, since is_visible return `false` by default, mocking it
to return false is not required.
This lets us simplify the long-ish ‘../../static/js’ paths, and will
remove the need for the ‘zrequire’ wrapper.
Signed-off-by: Anders Kaseorg <anders@zulip.com>