Remove Makefile

Remove the Makefile because:

  - it removes a dependency on `make(1)`
  - a lot of the tasks, e.g. `vagrant-up`, add little value
  - the `RUN` variable unnecessarily adds the `.+` regex; the default is
    to run all tests anyway
  - as we add more flags (i.e. vendor-specific flags), using a Makefile
    obscures these options
Skip 'install' step