DRTVWR-418: Remove a couple bad vendor branch tactic artifacts.
It seems that, perhaps due to mistakenly checking in past vendor updates on the default branch rather than the vendor branch, curl/lib/multi.c has acquired a couple bits of cruft carried over from previous releases. These are now causing compile errors. Try cleaning them out of multi.c in hopes it helps. GOOD_EASY_HANDLE is now defined in urldata.h, not locally in this file. isHandleAtHead() is dubious because there's no more SessionHandle struct.