remove old UNIX-specific Makefile

[christian-rauch]

I would like to remove the Makefile since:

1. it is not tested on the CI
2. does not compile the Python bindings
3. confuses users how to build and install the apriltag library ("No rule to make target '../libapriltag.a' " when compiling demo codes #100)

Also see the discussion here: #210.

[wxmerkt]

I am okay with this since it's out of sync with the CMake and leads to different behaviour. We may want to highlight explicitly in CHANGELOG and in the version bump that it's been removed in case some downstream workflow dependent on it without explicit version tagging (e.g. off master)