Remove unnecessary py.typed markers in python subpackages

[francesco-ballarin]

py.typed markers in subpackages are not necessary, see

Package maintainers who wish to support type checking of their code MUST add a marker file named py.typed to their package supporting typing. This marker applies recursively: if a top-level package includes it, all its sub-packages MUST support type checking as well.

in https://peps.python.org/pep-0561/#packaging-type-information

Tested on https://github.com/FEniCS/dolfinx/actions/runs/6674824040 for dolfinx,
and with multiphenics/multiphenicsx@c677f42 and https://github.com/multiphenics/multiphenicsx/actions/runs/6674840436 on multiphenicsx to ensure that typing of dolfinx does not break with downstream projects.