tests: Don't delete atconfig in clean
tests: Don't delete atconfig in clean

This file is created in ./configure so we shouldn't remove it with make.

Otherwise ./configure && make clean && make check fails
with:

make[3]: *** No rule to make target `atconfig', needed by `check-local'.
Stop.

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis,telenoobie/libosmo-abis,telenoobie/libosmo-abis