configure: remove multiple_arf experiment

the code tied to CONFIG_MULTIPLE_ARF was deleted in:
2611022 Clean out old CONFIG_MULTIPLE_ARF code.

Change-Id: Ie70bf047cde7e88d4b3996c8ff529e409bbe99e2
diff --git a/configure b/configure
index 604ad73..4ef4716 100755
--- a/configure
+++ b/configure
@@ -271,7 +271,6 @@
     unistd_h
 "
 EXPERIMENT_LIST="
-    multiple_arf
     spatial_svc
     denoising
     fp_mb_stats