Save memory less agressively

We can't drop the whole package set once the first solver iteration
finishes, because it runs for each arch separately and we need the data
for each of them. We can however delete the arch specific portion.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/phases/gather/methods/method_deps.py

@@ -146,7 +146,7 @@ def write_pungi_config(compose, arch, variant, packages, groups, filter_packages
                         '{0.name}-{1}:{0.version}-{0.release}'.format(rpm_obj, rpm_obj.epoch or 0))
 
         # Drop the packageset to hopefully save some memory
-        variant.pkgset = None
+        variant.pkgset.rpms_by_arch[arch] = None
 
     pungi_wrapper.write_kickstart(
         ks_path=pungi_cfg, repos=repos, groups=groups, packages=packages_str,