[interfaces] delete interfaces.hh

dune/xt/functions/ESV2007.hh

dune/xt/functions/base/combined.hh

dune/xt/functions/checkerboard.hh

dune/xt/functions/composition.hh

dune/xt/functions/constant.hh

dune/xt/functions/expression/default.hh

dune/xt/functions/expression/parametric.hh

dune/xt/functions/factories/functions.hh

dune/xt/functions/factories/grid-functions.hh

dune/xt/functions/flattop.hh

dune/xt/functions/indicator.hh

dune/xt/functions/interfaces.hh

python/dune/xt/functions/interfaces.hh