remove the obsolete 'm4' subdirectory

as far as I can see, it provided M4 macros for autotools based
projects which wanted to use OPM. I have severe doubts that this was
used by anyone because it most likely does not even work. (e.g. the M4
macros does not seem to specify the ERT and opm-parser prerequisites.)

Note that I'm not 100% sure of the above, so if I'm wrong, state so
loudly and I'll close the PR...