fix(language): remove rustlang "box" keyword (@Huterenok) (monkeytypegame#6397)

### Description

Removes Rust language "box" keyword, as it was removed a long time ago
in rust-lang/rust#49733

Author: Huterenok

frontend/static/languages/code_rust.json

@@ -112,7 +112,6 @@
     "to_lowercase()",
     "to_uppercase()",
     "replace(\"a\", \"b\")",
-    "box",
     "lazy_static",
     "iter()",
     "into_iter()",