flup: drop package

Latest release on pypi is from 2018
   https://pypi.org/project/flup/

The homepage looks outdated (~2005):
   https://www.saddi.com/software/flup/

No other package in the feeds uses this.

Time to retire this from the main branch.

PR for abandoned packages:
   openwrt/packages-abandoned#46

Signed-off-by: Alexandru Ardelean <[email protected]>