build: remove now-useless --{en, dis}able-tests

commit d771020 "don't build tests unless make check is run" has made the
 --{en,dis}able-tests switch completely useless.  The differentiation is
now made by running "make check" or not doing so.  The only effect of
the switch is an "empty" excursion of make into the tests/ directory.

(well, and it turns "make check" useless from the main directory if
 --disable-tests is given, which I don't think makes sense either)

Acked-by: Greg Troxel <gdt@ir.bbn.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/Makefile.am b/Makefile.am
index 37fea43..6916470 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -2,7 +2,7 @@
 
 SUBDIRS = lib @ZEBRA@ @BGPD@ @RIPD@ @RIPNGD@ @OSPFD@ @OSPF6D@ @BABELD@ \
          @ISISD@ @WATCHQUAGGA@ @VTYSH@ @OSPFCLIENT@ @DOC@ m4 @pkgsrcdir@ \
-         redhat @SOLARIS@ @BUILD_TESTS@
+         redhat @SOLARIS@ tests
 
 DIST_SUBDIRS = lib zebra bgpd ripd ripngd ospfd ospf6d babeld \
 	  isisd watchquagga vtysh ospfclient doc m4 pkgsrc redhat tests \