[Impeller] Remove validation log when the pipeline library is collected before pipeline is setup. (#38306)

This is a benign condition but the validation logs could be fatal since
8744c93c4d5. Don't crash in these
situations.

Author: chinmaygarde

impeller/renderer/backend/metal/pipeline_library_mtl.mm

@@ -119,8 +119,6 @@
 
         auto strong_this = weak_this.lock();
         if (!strong_this) {
-          VALIDATION_LOG << "Library was collected before a pending pipeline "
-                            "creation could finish.";
           promise->set_value(nullptr);
           return;
         }