Remove obsolete documentation about drop-flags

src/drop-flags.md

@@ -79,17 +79,5 @@ if condition {
 }
 ```
 
-As of Rust 1.0, the drop flags are actually not-so-secretly stashed in a hidden
+The drop flags are tracked on the stack and no longer stashed in types that
-field of any type that implements Drop. Rust sets the drop flag by overwriting
+implement drop.
-the entire value with a particular bit pattern. This is pretty obviously Not
-The Fastest and causes a bunch of trouble with optimizing code. It's legacy from
-a time when you could do much more complex conditional initialization.
-
-As such work is currently under way to move the flags out onto the stack frame
-where they more reasonably belong. Unfortunately, this work will take some time
-as it requires fairly substantial changes to the compiler.
-
-Regardless, Rust programs don't need to worry about uninitialized values on
-the stack for correctness. Although they might care for performance. Thankfully,
-Rust makes it easy to take control here! Uninitialized values are there, and
-you can work with them in Safe Rust, but you're never in danger.