[DAGCombine] Remove some old dead code for forming SETCC nodes

This code was dead when it was committed in r23665 (Oct 7, 2005), and before it
reaches its 10th anniversary, it really should go. We can always bring it back
if we'd like, but it forms more SETCC nodes, and the way we do legality
checking on SETCC nodes is wrong in a number of places, and removing this means
fewer places to fix. NFC.

llvm-svn: 246466