[ng] cleanup: remove two almost-unused global vars: {am,in}_file_name

[ng] cleanup: remove two almost-unused global vars: {am,in}_file_name

* automake.in ($am_file_name, $in_file_name): Delete these, which were
used only in the 'read_main_am_file' subroutine; instead ...
(read_main_am_file): ... modify it to only work from the '$makefile_am'
argument (which it was already receiving), and the new '$makefile_in'
argument, which is now passed to it ...
(generate_makefile): ... from here.
(initialize_per_input): Don't reset the two deleted variables anymore.

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