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.