Remove HEX_MIRROR uses

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

docker/multiarch/Dockerfile

@@ -32,7 +32,7 @@ WORKDIR /mobilizon
 
 # Elixir release
 RUN source /root/.bashrc && \
-    HEX_MIRROR="https://cdn.jsdelivr.net/hex" mix deps.get --only prod && \
+    mix deps.get --only prod && \
     mix compile  && \
     mix phx.digest.clean --all && \
     mix release --path release/mobilizon && \

docker/production/Dockerfile

@@ -20,7 +20,7 @@ COPY mix.exs mix.lock ./
 ENV MIX_ENV=prod
 RUN mix local.hex --force \
     && mix local.rebar --force \
-    && HEX_MIRROR="https://cdn.jsdelivr.net/hex" mix deps.get
+    && mix deps.get
 
 COPY lib ./lib
 COPY priv ./priv