Remove splice page from unstable book.

src/doc/unstable-book/src/library-features/splice.md

@@ -1,22 +0,0 @@
-# `splice`
-
-The tracking issue for this feature is: [#44643]
-
-[#44643]: https://github.com/rust-lang/rust/issues/44643
-
-------------------------
-
-The `splice()` method on `String` allows you to replace a range
-of values in a string with another range of values.
-
-A simple example:
-
-```rust
-#![feature(splice)]
-let mut s = String::from("α is alpha, β is beta");
-let beta_offset = s.find('β').unwrap_or(s.len());
-
-// Replace the range up until the β from the string
-s.splice(..beta_offset, "Α is capital alpha; ");
-assert_eq!(s, "Α is capital alpha; β is beta");
-```