Revert "Reinstate [SILGen] Implement NSString -> String bridging through _ObjectiveCBridgeable."
Revert "Reinstate [SILGen] Implement NSString -> String bridging through _ObjectiveCBridgeable."

This reverts commit a51367c569014279ede1b8675644a3cfad88d5ee. The
builders are failing in ways that are clearly related to my changes
but I'm unable to replicate locally.

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation