gnu: Remove rust-downcast-0.10.

* gnu/packages/crates-io.scm (rust-downcast-0.10): Delete variable.

gnu/packages/crates-io.scm

@@ -17756,27 +17756,6 @@ from macros.")
 @code{Cargo.toml}.")
     (license (list license:expat license:asl2.0))))
 
-(define-public rust-downcast-0.10
-  (package
-    (name "rust-downcast")
-    (version "0.10.0")
-    (source
-     (origin
-       (method url-fetch)
-       (uri (crate-uri "downcast" version))
-       (file-name (string-append name "-" version ".tar.gz"))
-       (sha256
-        (base32 "07bh0l95gwrzak6rj29v8kkm577d8vivxsxhqgscf64b4bq59d2b"))))
-    (build-system cargo-build-system)
-    (arguments `(#:skip-build? #t))
-    (home-page "https://github.com/fkoep/downcast-rs")
-    (synopsis
-     "Trait for downcasting trait objects back to their original types")
-    (description
-     "This crate provides a trait for downcasting trait objects back to their
-original types.")
-    (license license:expat)))
-
 (define-public rust-downcast-rs-1
   (package
     (name "rust-downcast-rs")