[InlineCost] Remove a dead constant; NFC

My `grep`-fu indicates that this hasn't been used for years. It also no
longer makes much sense to have this flavor of penalty in general, since
a call to a noreturn should mean that we're in a BB that's terminated by
`unreachable`. That case is accounted for by
CallAnalyzer::allowSizeGrowth.

llvm-svn: 345789