SIL: Remove an obsolete verifier check

This code has been commented out for years, doesn't compile,
and when I changed it to compile it blew up building overlays.

I don't feel it adds any value to have it around so I'm
removing it.

Author: slavapestov

lib/SIL/SILVerifier.cpp

@@ -3903,18 +3903,6 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
   void visitSILFunction(SILFunction *F) {
     PrettyStackTraceSILFunction stackTrace("verifying", F);
 
-    if (F->getLinkage() == SILLinkage::PrivateExternal) {
-      // FIXME: uncomment these checks.
-      // <rdar://problem/18635841> SILGen can create non-fragile external
-      // private_external declarations
-      //
-      // assert(!isExternalDeclaration() &&
-      //        "PrivateExternal should not be an external declaration");
-      // assert(isFragile() &&
-      //        "PrivateExternal should be fragile (otherwise, how did it appear "
-      //        "in this module?)");
-    }
-
     CanSILFunctionType FTy = F->getLoweredFunctionType();
     verifySILFunctionType(FTy);