apps/bttester: disable EATT in default config

EATT causes issues with tests GAP/SEC/SEM/BV-{65-67}-C. It is
experimental feature and should not influence config for current test plan.
diff --git a/apps/bttester/syscfg.yml b/apps/bttester/syscfg.yml
index 4bd16fc..5db9af2 100644
--- a/apps/bttester/syscfg.yml
+++ b/apps/bttester/syscfg.yml
@@ -144,5 +144,3 @@
 
     BLE_MESH_ADV_BUF_COUNT: 20
     BLE_MESH_TX_SEG_MAX: 6
-
-    BLE_EATT_CHAN_NUM: 1