Remove pkg-config branch on find_dune_package
It is stablished that finding dune modules with pkg-config does not work #188. To help advance on other regards like super builds !1391 (closed), and to avoid the extra unnecessary verbosity it adds to the build #366 (comment 137407), I propose to remove it until it gets fixed with !910 or similar.
Edited by Santiago Ospina De Los Ríos