Rust: Convert split_off QL-defined barrier to a neutral model (which was always the intent).

rust/ql/lib/codeql/rust/frameworks/stdlib/alloc.model.yml

@@ -64,3 +64,5 @@ extensions:
       - ["<alloc::collections::vec_deque::VecDeque as core::convert::From>::from", "sink", "manual"]
       - ["<alloc::sync::Arc as core::convert::From>::from", "sink", "manual"]
       - ["<alloc::rc::Rc as core::convert::From>::from", "sink", "manual"]
+      - ["<alloc::string::String>::split_off", "sink", "manual"]
+      - ["<alloc::vec::Vec>::split_off", "sink", "manual"]

rust/ql/lib/codeql/rust/security/UncontrolledAllocationSizeExtensions.qll

@@ -63,17 +63,4 @@ module UncontrolledAllocationSize {
         branch = false
       )
   }
-
-  /**
-   * A barrier for uncontrolled allocation size flow into particular functions.
-   */
-  private class ModeledBarrier extends Barrier {
-    ModeledBarrier() {
-      exists(MethodCall c |
-        c.getStaticTarget().getCanonicalPath() =
-          ["<alloc::string::String>::split_off", "<alloc::vec::Vec>::split_off"] and
-        this.asExpr() = c.getAnArgument()
-      )
-    }
-  }
 }