removed unstable description

Author: fneddy

src/doc/rustc/src/codegen-options/index.md

@@ -487,11 +487,6 @@ one of the following values:
 If not specified, overflow checks are enabled if
 [debug-assertions](#debug-assertions) are enabled, disabled otherwise.
 
-## packed-stack
-
-This flag enables packed StackFrames on s390x. Enabling both `packed-stack` and `backchain` attributes is incompatible
-with the default s390x target. Switch to s390x-unknown-none-softfloat if you need both attributes.
-
 ## panic
 
 This option lets you control what happens when the code panics.