Remove transfer_markers

[Tadaboody]

Closes #104
Tests still pass - am I missing anything?

[RonnyPfannschmidt]

this is "broken" for pytest versions before the marker fixup which made marker transfer unnecessary

its possible that this detail is not actually tested in detail as it was arcane and broken before the marker fixup

for correctness transfer_markers should be declared as a noop when its missing

im not sure how to correctly test this one

[Tinche]

Thanks for this contribution. @RonnyPfannschmidt does the PR look ok?

[RonnyPfannschmidt]

Looks okish now, unfortunately a test might need too much gymnastics right now (pytests fault)

[Tinche]

Thanks! Will cut a release later tonight.