[ng] cleanup: remove obsolete special-handling of default source for libs

[ng] cleanup: remove obsolete special-handling of default source for libs

Since we are already breaking backward-compatibility in small ways in
Automake-NG, we can snatch the occasion to do some cleanup of obsolescent
behaviours and backward-compatibility hacks.  This is the first of such
cleanups.

* automake.in (handle_source_transform): The default source for libfoo.la
is libfoo.c, but for backward compatibility we used look for libfoo_la.c
first.  Don't do that anymore now.
* t/ltlibsrc.sh: Remove as obsolete.
* NG-NEWS: Update.

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