cleanup: get rid of 'Automake::Configure_ac' module

cleanup: get rid of 'Automake::Configure_ac' module

Now that we unconditionally assume the Autoconf input is named
configure.ac, it's easier and clearer to inline the checks on
the existence of such a file directly in the automake and
aclocal scripts.

* automake.in (Automake::Configure_ac): Drop this import.
(Main code): Check the existence of configure.ac directly, instead
of calling the noe-removed 'require_configure_ac' function.
* aclocal.in: Likewise.

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