Do not store aports directory in config file (#247)

This addresses #246 by not storing the aports directory in the config
file. The default location is still available (from config/__init__.py),
and can be overridden.

pmb/config/init.py

@@ -63,6 +63,9 @@ def init(args):
                                  default=default)
     cfg["pmbootstrap"]["timestamp_based_rebuild"] = str(answer == "y")
 
+    # Do not save aports location to config file
+    del cfg["pmbootstrap"]["aports"]
+
     # Save config
     pmb.config.save(args, cfg)