Remove obsolete RustConfig file (#6455)

`RustConfig` was needed while we didn't have a `rust-toolchain.toml` file, but since that got added now, we don't need the `RustConfig` file anymore (see https://github.com/emk/heroku-buildpack-rust/tree/cfa0f0621411f0dbb46df9d4740e119b7a459ccc#specifying-which-version-of-rust-to-use)

Author: Turbo87

.github/renovate.json5

@@ -19,15 +19,6 @@
     "rust": {
         "labels": ["A-backend ⚙️"],
     },
-    "regexManagers": [
-        {
-            "fileMatch": ["^RustConfig$"],
-            "matchStrings": ["VERSION=(?<currentValue>.*?)\n"],
-            "depNameTemplate": "rust",
-            "datasourceTemplate": "github-releases",
-            "lookupNameTemplate": "rust-lang/rust",
-        }
-    ],
     "packageRules": [{
         "matchPackageNames": ["ember-cli", "ember-data", "ember-source"],
         "separateMinorPatch": true,

.github/workflows/ci.yml

@@ -51,7 +51,6 @@ jobs:
             build.rs
             Cargo.lock
             Cargo.toml
-            RustConfig
             rust-toolchain.toml
 
     outputs: