[ng] cleanup: remove 'clean-hdr.am'

[ng] cleanup: remove 'clean-hdr.am'

It's so small and dumb that it's easier and cleaner to just inline
it in the automake script.

* lib/am/clean-hdr.am: Delete.
* Makefile.am (dist_am_DATA): Remove it.
* automake.in (handle_configure): Just add the header configs and their
stamp files to '%clean_files', instead of processing the now-removed
'clean-hdr.am' to obtain the same effect.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>