feat: always use Sourcegraph extensions, remove feature flag

[sqs]

Removes the feature flag to toggle between whether to use Sourcegraph extensions and not. Sourcegraph extensions will always be used.

@ijsnow I opened this up per https://sourcegraph.slack.com/archives/C07KZF47K/p1540456636000100?thread_ts=1540453431.000100&cid=C07KZF47K. What else is necessary before we can merge it? Are there any other old code paths to remove?

Feel free to take over this PR.

[ijsnow]

This looks good for now. There will be a lot more code removed once all code hosts have extension support and implement the CodeHost interface.

[ijsnow]

Rethinking, we should consider waiting until I get at least the second bullet point done here https://github.com/sourcegraph/browser-extensions/issues/273.

It looks like e2e tests passed in CI but I've ran them 4 times locally now and they've failed 1/4 times in different places. Could be due to wake up time for the lang server extensions?

[chrismwendt]

Here's how to move this PR to https://github.com/sourcegraph/sourcegraph/tree/master/packages/browser-extensions

cd browser-extensions
git format-patch master --stdout > /tmp/patch

cd ../sourcegraph
cat /tmp/patch | git am -3 --directory=packages/browser-extensions/
# and fixup merge conflicts