build: Remove some unused variables from Makefile.real.

build: Remove some unused variables from Makefile.real.

All executables are converted, so $converted_executables is the same
as $executables, modulo executables which can not be built (and are
thus irrelevant). Similarly, $unconverted_executables is empty and
can be removed as well.

The $m4_deps variable was only used for the gengetopt files, so this
can also be removed, along with $ggo_dir.

$help2man_dir used to contain the path to the directory for the output
files of help2man. Since we now generate the man pages with lopsubgen,
$help2man_dir has become unused and can be removed.