Remove "exported" ninja variables

There was infrastructure to export ninja variables to bazel. Now that
the bazel migration is cancelled, we don't need it anymore.

Bug: 315353489
Test: m nothing
Change-Id: I298cc2ac7ebd004557be3b30d75f7357cab0b7a0
diff --git a/rust/config/arm_device.go b/rust/config/arm_device.go
index a5f4afb..5394e8a 100644
--- a/rust/config/arm_device.go
+++ b/rust/config/arm_device.go
@@ -44,7 +44,7 @@
 			strings.Join(rustFlags, " "))
 	}
 
-	ExportedVars.ExportStringListStaticVariable("DEVICE_ARM_RUSTC_FLAGS", ArmRustFlags)
+	pctx.StaticVariable("DEVICE_ARM_RUSTC_FLAGS", strings.Join(ArmRustFlags, " "))
 }
 
 type toolchainArm struct {