🔥(backend) remove rust from the back-builder Dockerfile stage

Rust was installed to build the pycrdt wheel. There is no need anymore
to install rust, a wheel is available for this package.

Dockerfile

@@ -14,13 +14,6 @@ FROM base AS back-builder
 
 WORKDIR /builder
 
-# Install Rust and Cargo using Alpine's package manager
-RUN apk add --no-cache \
-  build-base \
-  libffi-dev \
-  rust \
-  cargo
-
 # Copy required python dependencies
 COPY ./src/backend /builder