fix: remove autoconf features default value in help message

fix: remove autoconf features default value in help message

The default values of yes|no can be confusing combined with the
--enable / --disable switches of autoconf, remove them from the help
message.

Before :

  --disable-man-pages     Do not build and install man pages (already built in
                          a distributed tarball
                          [default=yes]

After :

  --disable-man-pages     Do not build and install man pages (already built in
                          a distributed tarball

Change-Id: I7a290aaacbcd0d04e0efd161357755451e5d4d4d
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/6742
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>