Rollback multi-rust version changes to the dockerfile (#2852)

## Motivation and Context
solved by deploying a newer version of smithy-rs instead

----

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tools/ci-build/Dockerfile

@@ -6,7 +6,6 @@
 # This is the base Docker build image used by CI
 
 ARG base_image=public.ecr.aws/amazonlinux/amazonlinux:2
-ARG prev_rust_stable_version=1.67.1
 ARG rust_stable_version=1.68.2
 ARG rust_nightly_version=nightly-2023-05-31
 
@@ -26,7 +25,6 @@ RUN curl https://musl.libc.org/releases/musl-1.2.3.tar.gz -o musl-1.2.3.tar.gz \
 FROM bare_base_image AS install_rust
 ARG rust_stable_version
 ARG rust_nightly_version
-ARG prev_rust_stable_version
 ENV RUSTUP_HOME=/opt/rustup \
  CARGO_HOME=/opt/cargo \
  PATH=/opt/cargo/bin/:${PATH} \
@@ -62,7 +60,6 @@ RUN set -eux; \
  rustup component add rustfmt; \
  rustup component add clippy; \
  rustup toolchain install ${rust_nightly_version} --component clippy; \
- rustup toolchain install ${prev_rust_stable_version} --component clippy; \
  rustup target add x86_64-unknown-linux-musl; \
  rustup target add wasm32-unknown-unknown; \
  rustup target add wasm32-wasi; \