support/config-fragments/autobuild: stop testing Sourcery AMD64 toolchain

support/config-fragments/autobuild: stop testing Sourcery AMD64 toolchain

This toolchain uses an old gcc 6.2.0, and newer versions of the
toolchain are no longer publicly available. This old gcc 6.2.0 causes
build issues of Boost, which are unfixable without updating the
toolchain. As we're about to drop support for this toolchain entirely,
we must stop testing it in our autobuilder infrastructure.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>