veyron/runtimes/google/ipc: Always allow MethodSignature
veyron/runtimes/google/ipc: Always allow MethodSignature

Before this change, all requests to MethodSignature would be rejected
because there is no MethodTags in the security context. For now, we
decided to allow all requests, i.e. remove the call to authorize().

Added a TODO to decide if we should hide method signatures that the
caller doesn't have access to.

Change-Id: Ib89f3e2f36f3aa7d29f157b883eb4236ee139dbd

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref