media-video/smplayer: Remove youtube-dl runtime dependency

media-video/smplayer: Remove youtube-dl runtime dependency

The net-misc/youtube-dl package is not a required dependency to use
media-video/mpv as a backend for smplayer. This patch removes the
net-misc/youtube-dl package RDEPEND line and adds an informative
pkg_postinst message instead to advise the user to install the
net-misc/youtube-dl package for URL support.

Closes: https://bugs.gentoo.org/649184
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Closes: https://github.com/gentoo/gentoo/pull/11223
Signed-off-by: Lars Wendler <polynomial-c@gentoo.org>