Remove fluent-web (#261)

fluent-web landed in June 2017 in #44. It was an example of an
end-to-end localization solution for the Web based on Fluent.

Since then we've focused on fluent-dom which is used in Firefox,
and on fluent-react used by a number of Mozilla and non-Mozilla
websites.

fluent-web has had 0 releases in the last year. It's not used by any
Mozilla project. It's also currently broken, which we don't even know
about, because fluent-web has 0 tests.

And yet, every maintenance task that we perform in the repo must also
account for the existence of fluent-web. This adds a maintenance burden
without much benefit.

Author: stasm