ci: drop check-warnings test

We already run `cargo build` with `RUSTFlAGS=-Dwarnings`, so also
running `cargo check` with the same arguments, on the same
architectures, and with the same rustflags gains us nothing.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

.buildkite/test_description.json

@@ -80,15 +80,6 @@
         "riscv64"
       ]
     },
-    {
-      "test_name": "check-warnings",
-      "command": "RUSTFLAGS=\"-D warnings\" cargo check --all-targets --all-features --workspace",
-      "platform": [
-        "x86_64",
-        "aarch64",
-        "riscv64"
-      ]
-    },
     {
       "test_name": "coverage",
       "command": "pytest $(find . -type f -name \"test_coverage.py\")",