automake: stop supporting "%KEY?iftrue:iffalse%" transforms

automake: stop supporting "%KEY?iftrue:iffalse%" transforms

And the similar "reduced forms" "%KEY:iffalse%" and "%KEY?iftrue%"
as well.

They are convoluted, never used (after the previous change), and will
get in the way in the Automake-NG branch, where we'll soon want to use
GNU make static pattern rules in our internal '*.am' fragments.

* automake.in (preprocess_file, transform): Simplify not to support
those transform patterns.

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