[ng] cleanup: merge '%compile_clean_files' in '%clean_files'

[ng] cleanup: merge '%compile_clean_files' in '%clean_files'

No need to keep them separated anymore.

* automake.in (%compile_clean_files): Delete.
(initialize_per_input): Don't reset it.
(handle_compile): Don't merge '%compile_clean_files'
contents into '%clean_files'.
(handle_single_transform): Update '%clean_files', not
'%compile_clean_files'.
(handle_LIBOBJS_or_ALLOCA): Likewise.

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