Remove all *.dir_list files and associated variables

* The list of Makefiles is automatically generated by autogen.sh. So
  there is no need to have manually updated dir_list files anymore.

* Some variables in Makefiles relied on these dir list files but did not
  appear to do anything (and I checked with Matthias) so they have been
  removed. The variables are:

    USER_DRIVER_SUBDIRECTORIES
    USER_SRC_SUBDIRECTORIES
    PRIVATE_DIR
    HAVE_PRIVATE_USER_SRC
    PRIVATE_SUBDIRECTORIES
    PRIVATE_USER_DRIVER_SUBDIRECTORIES
    HAVE_PRIVATE_USER_DRIVERS

* The variable HAVE_PRIVATE_DIRECTORY was used to decide if we should
  recurse into ./private when running make clean etc. I've replaced this
  by just running the command with a `-` prepended (so that it is
  allowed to fail without stopping make).

  Unfortunately I can't test it still works in exactly the same way
  because I've never used the private folder.