Remove manifest Rust/rustup (#4414)

Reasoning:
1. `winget install Rust/rustup` doesn't work
2. The exe it downloads doesn't seem to be "signed" which opens security
   box on windows
3. Even after choosing "continue anyway" on the warning box, winget
   doesn't install it anyway.

manifests/Rust/rustup/1.21.1.yaml

@@ -1,19 +0,0 @@
-Id: Rust.rustup
-Publisher: Rust
-Name: rustup
-Version: 1.21.1
-AppMoniker: rustup
-Description: Manage multiple rust installations with ease
-Tags: rust, rustlang, rustup, multirust, install, proxy, toolchain
-Homepage: https://github.com/rust-lang/rustup
-MinOSVersion: 10.0.0.0
-License: MIT or Apache-2.0
-LicenseUrl: https://opensource.org/licenses/MIT or https://opensource.org/licenses/Apache-2.0
-Installers:
-  - Arch: x64
-    Url: https://static.rust-lang.org/rustup/archive/1.21.1/x86_64-pc-windows-gnu/rustup-init.exe
-    Sha256: D17DF34BA974B9B19CF5C75883A95475AA22DDC364591D75D174090D55711C72
-    InstallerType: exe
-    Switches:
-        Silent: "-y --quiet"
-        SilentWithProgress: "-y"