Remove deprecated BOARD_BUILD_SYSTEM_ROOT_IMAGE

This variable is always false or empty now, so clean up remaining
references of the variable.

Bug: 241346584
Test: Presubmit
Change-Id: Id5c37a1391239206477fd802d364a2678692f513
diff --git a/init/Android.bp b/init/Android.bp
index f0e362c..f6f1e8c 100644
--- a/init/Android.bp
+++ b/init/Android.bp
@@ -308,7 +308,7 @@
     name: "init_first_stage_cc_defaults",
     module_type: "cc_defaults",
     config_namespace: "ANDROID",
-    bool_variables: ["BOARD_BUILD_SYSTEM_ROOT_IMAGE", "BOARD_USES_RECOVERY_AS_BOOT"],
+    bool_variables: ["BOARD_USES_RECOVERY_AS_BOOT"],
     properties: ["installable"],
 }
 
@@ -317,9 +317,6 @@
 init_first_stage_cc_defaults {
     name: "init_first_stage_defaults",
     soong_config_variables: {
-        BOARD_BUILD_SYSTEM_ROOT_IMAGE: {
-            installable: false,
-        },
         BOARD_USES_RECOVERY_AS_BOOT: {
             installable: false,
         },