Makefile: Remove duplicate cva6_config_pkg

Created by: niwis

cva6_config_pkg is currently listed in tbs and src. Elaborating it twice causes some tools to throw a warning (e.g. verilator) and others to error (e.g. questa). Fix this by removing cva6_config_pkg from the testbench file list (it is anyways used by the design and thus a design source).