drop the separate build-dependencies step

Since we don't run the unit tests as part of the package build now we don't
need it

.circleci/config.yml

@@ -591,21 +591,6 @@ commands:
               -p 'python3.withPackages (ps: [ ps.setuptools ])' \
               --run 'python setup.py update_version'
 
-      - "run":
-          name: "Build Dependencies"
-          command: |
-            source .circleci/lib.sh
-
-            NIXPKGS=$(nixpkgs_flake_reference nixpkgs-<<parameters.nixpkgs>>)
-
-            # `nix develop` will build all of the dependencies of the target but
-            # not the target itself.
-            cache_if_able nix develop \
-              --command "true" \
-              --cores $DEPENDENCY_CORES \
-              --override-input nixpkgs "$NIXPKGS" \
-              .#<<parameters.pythonVersion>>-tahoe-lafs
-
       - "run":
           name: "Build Package"
           command: |