Remove feature const_ptr_offset

Summary: Another feature we don't use.

Reviewed By: bobyangyf

Differential Revision: D41015661

fbshipit-source-id: 7f73bc9b47e5e8d6008247026cc6ae8fe8864ac7

Author: stepancheg

starlark/src/lib.rs

@@ -351,7 +351,6 @@
 #![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(const_mut_refs)]
-#![feature(const_ptr_offset)]
 #![feature(const_type_id)]
 #![feature(core_intrinsics)]
 #![feature(generic_associated_types)]