Remove support for *.expect files in regressions (#2341)

Remove support for *.expect files in regressions (#2341)

Currently, we can optionally specify an *.expect file with the metadata
of a regression test. This commit removes that option because it was not
widely used, adds maintenance overhead and makes the transition to a new
build system more cumbersome. Regression files can still be fed to a
solver without removing the metadata first since they are in comments of
the corresponding input format (note that this was not always the case,
it changed in efc6163629c6c5de446eccfe81777c93829995d5).