Remove FIXME that I already fixed in e02266d.

Author: gottesmm

include/swift/SIL/Projection.h

@@ -838,9 +838,6 @@ class ProjectionTree {
   SILModule &Mod;
 
   /// The allocator we use to allocate ProjectionTreeNodes in the tree.
-  ///
-  /// FIXME: This should be a reference to an outside allocator. We shouldn't
-  /// have each ProjectionTree have its own allocator.
   llvm::SpecificBumpPtrAllocator<ProjectionTreeNode> &Allocator;
 
   // A common pattern is a 3 field struct.