Dockerfile: remove old, unnnecessary, conflicting fetching of Go

The Makefile's toolchain target fetches the right Go version
nowadays. We can make the Dockerfile do that and cache it later, but
for now this at least prevents the Go version mismatch bugs.

Dockerfile

@@ -30,23 +30,12 @@ RUN echo y | $ANDROID_HOME/tools/bin/sdkmanager 'ndk;20.0.5594570'
 RUN echo y | $ANDROID_HOME/tools/bin/sdkmanager 'platform-tools'
 RUN echo y | $ANDROID_HOME/tools/bin/sdkmanager 'build-tools;28.0.3'
 
-# Get Go stable release
-WORKDIR $HOME
-ARG GO_VERSION=1.16.5
-RUN curl -O https://storage.googleapis.com/golang/go${GO_VERSION}.linux-amd64.tar.gz
-RUN echo "b12c23023b68de22f74c0524f10b753e7b08b1504cb7e417eccebdd3fae49061  go${GO_VERSION}.linux-amd64.tar.gz" | sha256sum -c
-RUN tar -xzf go${GO_VERSION}.linux-amd64.tar.gz && mv go goroot
-ENV GOROOT $HOME/goroot
-ENV PATH $PATH:$GOROOT/bin:$HOME/bin:$ANDROID_HOME/platform-tools
+ENV PATH $PATH:$HOME/bin:$ANDROID_HOME/platform-tools
 ENV ANDROID_SDK_ROOT /build/android-sdk
 
 RUN mkdir -p $HOME/tailscale-android
 WORKDIR $HOME/tailscale-android
 
-ADD go.mod .
-ADD go.sum .
-RUN go mod download
-
 # Preload Gradle
 COPY android/gradlew android/gradlew
 COPY android/gradle android/gradle