configs/ts4800: remove defconfig

This defconfig has been failing to build since we switched the default
gcc version to gcc 8.x, as the Linux kernel version is too old and
doesn't contain the necessary fixes to build with gcc >= 8.x.

Despite several pings to the original submitter of the defconfig
(which is not listed in MAINTAINERS), no fix has been sent, so it is
time to drop this defconfig before the 2019.08 release.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/278489442



Cc: Patrick Keroulas <patrick.keroulas@savoirfairelinux.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>