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/x86_device.go b/rust/config/x86_device.go
index 2a57e73..5d9d88a 100644
--- a/rust/config/x86_device.go
+++ b/rust/config/x86_device.go
@@ -56,7 +56,7 @@
 			strings.Join(rustFlags, " "))
 	}
 
-	ExportedVars.ExportStringListStaticVariable("DEVICE_X86_RUSTC_FLAGS", x86RustFlags)
+	pctx.StaticVariable("DEVICE_X86_RUSTC_FLAGS", strings.Join(x86RustFlags, " "))
 }
 
 type toolchainX86 struct {