NEW_MULTISYMBOL: make experiment depend on EC_ADAPT.

Simplify the number of different compile and code paths.

Change-Id: Id5b061571daeb070b91ec762fe7e07e8c114229e

configure

diff --git a/configure b/configure
index a3cd03f..0f511ed 100755
--- a/configure
+++ b/configure
@@ -531,6 +531,7 @@
     enabled dpcm_intra && enable_feature ext_tx
     enabled chroma_sub8x8 && enable_feature cb4x4
     enabled compound_singleref && enable_feature ext_inter
+    enabled new_multisymbol && enable_feature ec_adapt
 
     if ! enabled delta_q && enabled ext_delta_q; then
       log_echo "ext_delta_q requires delta_q, so disabling ext_delta_q"