Relax the check for dexpreopt files in the ART apex.

This is to fix builds that set WITH_DEXPREOPT=false variable in Make.

Test: aosp_walleye-userdebug builds.
Test: aosp_walleye-eng builds with a temporary patch in
  build/make/core/board_config.mk that disables dexpreopt:
    -ifeq ($(HOST_OS),linux)
    -  WITH_DEXPREOPT := true
    -endif
    +# ifeq ($(HOST_OS),linux)
    +#   WITH_DEXPREOPT := true
    +# endif

Bug: 149273451

Change-Id: I376e01760ea238b1f45f7e236d3c391c1d068c23
diff --git a/build/apex/art_apex_test.py b/build/apex/art_apex_test.py
index d80db3c..f2e2734 100755
--- a/build/apex/art_apex_test.py
+++ b/build/apex/art_apex_test.py
@@ -347,8 +347,6 @@
 
   def check_dexpreopt(self, basename):
     dirs = self.arch_dirs_for_path('javalib')
-    if not dirs:
-      self.fail('Could not find javalib directory for any arch.')
     for dir in dirs:
       for ext in ['art', 'oat', 'vdex']:
         self.check_file('%s/%s.%s' % (dir, basename, ext))