Auto merge of #463 - JustForFun88:remove_duplicate, r=Amanieu

Remove duplicate job

This part was added in #347 to keep one CI builder at 1.56.1 for compatibility, but it's no longer relevant and doesn't do anything.
I noticed this when I was dealing with #460 but forgot to fix it.

Author: bors

.github/workflows/rust.yml

@@ -73,9 +73,6 @@ jobs:
         - os: windows-latest
           target: x86_64-pc-windows-msvc
           channel: 1.64.0
-        - os: ubuntu-latest
-          target: x86_64-unknown-linux-gnu
-          channel: 1.64.0
         - os: ubuntu-latest
           target: x86_64-unknown-linux-gnu
           channel: beta