Remove *.egg-info directories in clean step

Removing the egg-info directory automatically in the 'clean' step means that
if the build changes the egg-info, the change will not trip up the next
build of the source package.

* if the egg-info directory is generated by the build, then removing it
  is the right thing to do

* if the egg-info directory is included in the upstream source, then
  deleting it causes no harm as dpkg-source warns on missing files in
  the source package but does not exit with error.