[ng] refactoring: no "cleaning" rules in compile.am

[ng] refactoring: no "cleaning" rules in compile.am

* automake.in (handle_compile): To ensure the compiled objects are
removed, simply merge the contents of '%compile_clean_files' into
'%clean_files' here, instead of ...
* lib/am/compile.am (am__mostlyclean_files): ... appending them to
this variable here.  While we are at it, remove a stray declaration
of the already-deleted target 'mostlyclean-compile' as ".PHONY".

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