emsdk

Update some stale information in README.md (#1190)

- Remove reference to `~/.emscripten`.  We no longer use the home
  directory to store config information by default, either in
  emscripten or in emsdk.
- Remove some references to `The Emscripten Command Prompt`.  While I
  suppose this is referring to the windows-only `emcmdprompt.bat`, and I
  suppose it means "any shell where `activate` has been run", I think
  its more clear to simply avoid using the term.