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/cc/ndk_library.go b/cc/ndk_library.go
index 39dd415..25231fd 100644
--- a/cc/ndk_library.go
+++ b/cc/ndk_library.go
@@ -225,7 +225,7 @@
 }
 
 func init() {
-	config.ExportStringList("StubLibraryCompilerFlags", stubLibraryCompilerFlags)
+	pctx.StaticVariable("StubLibraryCompilerFlags", strings.Join(stubLibraryCompilerFlags, " "))
 }
 
 func addStubLibraryCompilerFlags(flags Flags) Flags {