[Gardening] Delete dead comment.

This comment referred to the now-deleted workaround
AvailabilityWithLeaks boundary kind.

Author: nate-chandler

include/swift/SIL/OSSALifetimeCompletion.h

@@ -63,9 +63,6 @@ class OSSALifetimeCompletion {
   // Availability: "As late as possible."  Consume the value in the last blocks
   //               beyond the non-consuming uses in which the value has been
   //               consumed on no incoming paths.
-  //
-  //                        This boundary works around bugs where SILGen emits
-  //                        illegal OSSA lifetimes.
   struct Boundary {
     enum Value : uint8_t {
       Liveness,