Sema: Remove unnecessary check

It looks like this is covered by the witness lookup below.

Kind of a useless change, but I'm trying to simplify the
LookupResultBuilder as much as I can.

Author: slavapestov

lib/Sema/TypeCheckNameLookup.cpp

@@ -146,10 +146,6 @@ namespace {
           return;
         }
 
-        // If we're validating the protocol recursively, bail out.
-        if (!foundProto->hasValidSignature())
-          return;
-
         // Dig out the witness.
         ValueDecl *witness = nullptr;
         auto concrete = conformance->getConcrete();