Remove unused optimizely

Remove unused optimizely

Here are the originating commits but I don't see much context, usage, why.

• https://gitlab.com/gitlab-org/gitter/webapp/-/commit/a1e7e6fcd1e5eb899cb1a5838467ee29cf77c8f4#1955ebf3cb6940a082be4cf12441c3379b039263_33_22
• https://gitlab.com/gitlab-org/gitter/webapp/-/commit/abd4a2901999cabb0867a039719a697db302df2e

On my browser, optimizely is already net::ERR_BLOCKED_BY_CLIENT from adblock.