[ng] maintcheck: remove a test that can't work for Automake-NG

[ng] maintcheck: remove a test that can't work for Automake-NG

* syntax-checks.mk (sc_tests_makefile_variable_order): Remove.  This
rule checked that all variables definitions were listed before rules
in the automake-generated Makefile.in files; but this invariant is not
guaranteed anymore by Automake-NG, and in fact it will break more and
more as our work proceeds.
(syntax_check_rules): Update.

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