Update makesrcdist: fix packaging bug

The previous version didn't delete the docs before the source files
were packed which resulted in too big release tarballs.

Notes:
 (1) Release 1.4.1 has been updated (files were replaced) so all
     further downloads are not affected.
 (2) The git commits on tag release-1.4.1 do not include this commit
     but this is OK. This will be in the next release.
 (3) makesrcdist itself is not packed in the source tarballs anyway.