Remove uninstallable Modem plugin

It depends on a package dreambox-modem-ppp-scripts that does not exist.
Remove it since nobody actually ever tried to install it for years.
Skip building DVDBurn plugin

DVDBurn depends on dvdauthor which doesn't compile, so skip building it
until that is fixed.
Merge branch 'master' of git://git.code.sf.net/p/openpli/enigma2