Remove vendored copy of fomantic-dropdown (#15193)

Remove vendored copy of fomantic-dropdown (#15193)

jQuery 3.6.0 seems to have broke the dropdown focus handling (focus
would get stuck on the dropdown) in this module which we have vendored
on top of fomantic for accessibility improvements.

Either downgrading jQuery to 3.5.1 or removing the vendor copy seems to
resolve the issue and I opted for removing the copy because I think such
changes should be done upstream and the removal also lightens the JS by
155kB before minify/gzip.

Fixes: https://github.com/go-gitea/gitea/issues/15172
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>