luci-theme-rosy: Remove BROKEN theme

The Rosy theme has been marked broken for over a year,
since 23 Dec 2019. Remove it now, so that it does not
get into the next release branch 20.xx (or 21.xx).

Reference to
https://github.com/openwrt/luci/issues/3759#issuecomment-599436159

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>