Remove empty core::lazy and std::lazy

PR #98165 with commits 7c360dc117 and c1a2db3372
has moved all of the components of these modules into different places,
namely {std,core}::sync and {std,core}::cell. The empty
modules remained. As they are unstable, we can simply remove them.

library/core/src/lib.rs

@@ -326,8 +326,6 @@ pub mod cell;
 pub mod char;
 pub mod ffi;
 pub mod iter;
-#[unstable(feature = "once_cell", issue = "74465")]
-pub mod lazy;
 pub mod option;
 pub mod panic;
 pub mod panicking;