Remove update-tags phase (#738)

Its not really very useful to cache the tot release in a local text
file.  Instead just fetch the revsion each time `install tot` is run.

This avoids folks accidentally installing "old" tot releases.

Also, make the output a little less chatty when looking up tot version.

docker/Dockerfile

@@ -23,7 +23,6 @@ COPY . ${EMSDK}
 
 RUN echo "## Install Emscripten" \
     && cd ${EMSDK} \
-    && if [ "$EMSCRIPTEN_VERSION" = "tot" ]; then ./emsdk update-tags; fi \
     && ./emsdk install ${EMSCRIPTEN_VERSION} \
     && echo "## Done"