Remove startup JS call for issue notes

This is not needed because this is a polling request that's supposed to
be triggered after 2.5 seconds.

This also wastes a lot of resources because this call does not send the
`X-Last-Fetched-At` header. So it is requesting all the notes of an
issue.

Changelog: performance

app/views/projects/issues/_discussion.html.haml

 - add_page_startup_api_call discussions_path(@issue)
-- add_page_startup_api_call notes_url
 
 - @gfm_form = true