By default, use smtk acquired from git

Dashboards are having difficulty building a developer build for cmb
because smtk defaults to being acquired "from-cmb" even if there is no
cmb to use.

versions.cmake

@@ -43,10 +43,10 @@ superbuild_set_selectable_source(smtk
   SELECT 2.0.0
     GIT_REPOSITORY  "https://gitlab.kitware.com/cmb/smtk.git"
     GIT_TAG         "v2.0.0"
-  SELECT git CUSTOMIZABLE
+  SELECT git CUSTOMIZABLE DEFAULT
     GIT_REPOSITORY  "https://gitlab.kitware.com/cmb/smtk.git"
     GIT_TAG         "origin/master"
-  SELECT from_cmb DEFAULT
+  SELECT from_cmb
     DOWNLOAD_COMMAND
       "${CMAKE_COMMAND}" -E echo_append
     SOURCE_DIR    "${smtk_source_dir}/thirdparty/smtk"