Revert "[linearelliptic.problems.default] extend ctor to take arbitrary functions"
Revert "[linearelliptic.problems.default] extend ctor to take arbitrary functions"

This reverts commit c00f6786aa3c142f0ba2ad261265d01fa86c3bcd.

tobiasleibner/dune-hdd,pymor/dune-hdd,pymor/dune-hdd,tobiasleibner/dune-hdd