Remove old .cvsignore files

As the source code is managed in GIT and there is a .gitignore file in
the top level directory specifying file names to exclude from version
control, then the old per-directory .cvsignore files for CVS are
redundant.

Add the only missing and applicable entry from src/.cvsignore of '.libs'
to .gitignore and remove all the .cvsignore files.