mpd: remove version 0.19

mpd: remove version 0.19

MPD version 0.20 is affected by GCC bug 64735 and failes to build for
some archictuctures (NIOSII, ARMv4, ARMv5 and SPARCv8) with a GCC
toolchain less then version 7.

We added a version choice between 0.19 and 0.20 in Buildroot version
2017.02 as GCC 7 was not yet available at that time. This way, mpd could
still be build for those architectures.

As GCC 7 is now available in Buildroot remove the version choice for 0.19.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>