[config] Clean up unused import and message type

The unused import was causing warnings during proto compilation.

Change-Id: Ie2651fcc36a01a4d201e9d1109710566566793d1
Reviewed-on: https://fuchsia-review.googlesource.com/c/infra/recipes/+/1036753
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Fuchsia-Auto-Submit: Oliver Newman <olivernewman@google.com>
Reviewed-by: Haowei Wu <haowei@google.com>
diff --git a/recipes/contrib/clang.proto b/recipes/contrib/clang.proto
index f406af9..cc2e42e6 100644
--- a/recipes/contrib/clang.proto
+++ b/recipes/contrib/clang.proto
@@ -6,14 +6,6 @@
 
 package recipes.fuchsia.contrib.clang;
 
-import "google/protobuf/struct.proto";
-
-enum LTOMode {
-  FALSE = 0;
-  FULL = 1;
-  THIN = 2;
-}
-
 message InputProperties {
   // Git repository URL.
   string repository = 1;