[Zucchini] Remove CHECK for unknown exe.

Was causing ClusterFuzz crashes when an unknown exe type was returned.
Instead of crashing the runtime should just return kExeTypeUnknown and
let the runtime handle the problem as before.

Bug: 836803
Change-Id: I23841fc50025a9850ac322960069270cd3e524be
Reviewed-on: https://chromium-review.googlesource.com/1028212Reviewed-by: Samuel Huang <huangs@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@google.com>
Cr-Commit-Position: refs/heads/master@{#553552}

components/zucchini/image_utils.h

@@ -173,7 +173,6 @@ constexpr ExecutableType CastToExecutableType(uint32_t possible_exe_type) {
     case kExeTypeUnknown:
       return static_cast<ExecutableType>(possible_exe_type);
     default:
-      NOTREACHED() << "Unknown type.";
       return kExeTypeUnknown;
   }
 }