chore(releases): Remove feature flag

[armenzg]

It removes the feature flag from #80655.

We GAed this morning. I will wait a bit before removing this code.

[armenzg]

This test was to prevent normal releases from regressing with the new fix.

This is the test it duplicated:

sentry/tests/sentry/issues/endpoints/test_group_details.py

Lines 429 to 449 in 1413cbf

	def test_resolved_in_next_release_no_release(self):
	self.login_as(user=self.user)
	project = self.create_project()
	project.flags.has_releases = True
	project.save()
	event = self.store_event(data={}, project_id=project.id)
	group = event.group
	assert group is not None
	url = f"/api/0/organizations/{group.organization.slug}/issues/{group.id}/"
	response = self.client.put(url, data={"status": "resolvedInNextRelease"})
	assert response.status_code == 200, response.content
	# Refetch from DB to ensure the latest state is fetched
	group = Group.objects.get(id=group.id, project=group.project.id)
	assert group.status == GroupStatus.RESOLVED
	# no GroupResolution because there is no release
	assert not GroupResolution.objects.filter(group=group).exists()
	assert response.data["statusDetails"] == {}