Remove unusued param

Fixes GCC build

Change-Id: I0d89d48bdaa24f4199afa03197e8bbf1a0576b28
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/44803
Commit-Queue: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: David Neto <dneto@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/src/castable_test.cc b/src/castable_test.cc
index 1f3f546..a62bc3d 100644
--- a/src/castable_test.cc
+++ b/src/castable_test.cc
@@ -81,12 +81,12 @@
     return true;
   });
 
-  ASSERT_TRUE((frog->Is<Animal>([](const Animal* a) { return true; })));
-  ASSERT_FALSE((frog->Is<Animal>([](const Animal* a) { return false; })));
+  ASSERT_TRUE((frog->Is<Animal>([](const Animal*) { return true; })));
+  ASSERT_FALSE((frog->Is<Animal>([](const Animal*) { return false; })));
 
   // Predicate not called if cast is invalid
   auto expect_not_called = [] { FAIL() << "Should not be called"; };
-  ASSERT_FALSE((frog->Is<Bear>([&](const Animal* a) {
+  ASSERT_FALSE((frog->Is<Bear>([&](const Animal*) {
     expect_not_called();
     return true;
   })));
@@ -165,12 +165,12 @@
     return true;
   });
 
-  ASSERT_TRUE((frog->Is<Animal>([](const Animal* a) { return true; })));
-  ASSERT_FALSE((frog->Is<Animal>([](const Animal* a) { return false; })));
+  ASSERT_TRUE((frog->Is<Animal>([](const Animal*) { return true; })));
+  ASSERT_FALSE((frog->Is<Animal>([](const Animal*) { return false; })));
 
   // Predicate not called if cast is invalid
   auto expect_not_called = [] { FAIL() << "Should not be called"; };
-  ASSERT_FALSE((frog->Is<Bear>([&](const Animal* a) {
+  ASSERT_FALSE((frog->Is<Bear>([&](const Animal*) {
     expect_not_called();
     return true;
   })));