Update or remove invalid code links in book

Author: detrumi

book/src/plumbing/fetch.md

@@ -1,9 +1,5 @@
 # Fetch
 
-```rust,no_run,noplayground
-{{#include ../../../src/plumbing.rs:fetch}}
-```
-
 The `fetch` operation computes the value of a query. It prefers to reuse memoized values when it can.
 
 ## Input queries

book/src/plumbing/maybe_changed_after.md

@@ -1,9 +1,5 @@
 # Maybe changed after
 
-```rust,no_run,noplayground
-{{#include ../../../src/plumbing.rs:maybe_changed_after}}
-```
-
 The `maybe_changed_after` operation computes whether a query's value *may have changed* **after** the given revision. In other words, `Q.maybe_change_since(R)` is true if the value of the query `Q` may have changed in the revisions `(R+1)..R_now`, where `R_now` is the current revision. Note that it doesn't make sense to ask `maybe_changed_after(R_now)`.
 
 ## Input queries

book/src/plumbing/query_ops.md

@@ -1,14 +1,6 @@
 # Query operations
 
-Each of the query storage struct implements the `QueryStorageOps` trait found in the [`plumbing`] module:
-
-```rust,no_run,noplayground
-{{#include ../../../src/plumbing.rs:QueryStorageOps}}
-```
-
- which defines the basic operations that all queries support. The most important are these two:
+The most important basic operations that all queries support are:
 
 * [maybe changed after](./maybe_changed_after.md): Returns true if the value of the query (for the given key) may have changed since the given revision.
 * [Fetch](./fetch.md): Returns the up-to-date value for the given K (or an error in the case of an "unrecovered" cycle).
-
-[`plumbing`]: https://github.com/salsa-rs/salsa/blob/master/src/plumbing.rs