Cleanup: DIST_SUBDIRS is redundant when using AM conditionals

Cleanup: DIST_SUBDIRS is redundant when using AM conditionals

From automake's documentation[1] :

  If SUBDIRS is defined conditionally using Automake conditionals,
  Automake will define DIST_SUBDIRS automatically from the possible
  values of SUBDIRS in all conditions.

[1] https://www.gnu.org/software/automake/manual/html_node/SUBDIRS-vs-DIST_005fSUBDIRS.html

Change-Id: I8495f1f4452ccde4920ecd63bfd37de4eb10c281
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>