MNT: Remove explicit dependency on openslide-python and ghalton

They are now listed as dependencies of TIRL.

I have uploaded macOS and linux packages for TIRL to a temporary test channel:

https://users.fmrib.ox.ac.uk/~paulmc/test_channel/

Until these packages are added to the FSL conda channel, you can test the fslpython installation by adding this test channel to the fslpython_environment.yml file, before all other channels, i.e.:

name: fslpython
channels:
 - https://users.fmrib.ox.ac.uk/~paulmc/test_channel/
 - conda-forge
 - defaults
 - https://fsl.fmrib.ox.ac.uk/fsldownloads/fslconda/channel
dependencies:
...