fix: remove #![feature(ptr_wrapping_offset_from)]

`wrapping_offset_from` has been deprecated and removed. The actual use
was removed by eb752db, but `#![feature(...)]` remained in the source
code.

Author: yvt

src/constance/src/lib.rs

@@ -9,7 +9,6 @@
 #![feature(const_raw_ptr_deref)]
 #![feature(const_checked_int_methods)]
 #![feature(const_option)]
-#![feature(ptr_wrapping_offset_from)]
 #![feature(cfg_target_has_atomic)] // `#[cfg(target_has_atomic_load_store)]`
 #![feature(unsafe_block_in_unsafe_fn)] // `unsafe fn` doesn't imply `unsafe {}`
 #![feature(never_type)] // `!`