Remove the code from Function::dropAllReferences which replaced

Remove the code from Function::dropAllReferences which replaced
uses of the function's blocks with undef. This code isn't needed,
because BasicBlock's destructor handles such uses. Also, undef isn't
correct, since blockaddresses may still be used for comparisons
with null.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121170 91177308-0d34-0410-b5e6-96231b3b80d8