Remove git call as fallback version discovery

[VeckoTheGecko]

Now that Parcels is in the src folder, and now that we promote the use of Pixi (or pip install -e .) to do source installation of Parcels - I don't think we need git calls to determine the version number.

This does mean that the version number does not have the git sha in it when using dev versions

e.g.,

>>> parcels.__version__
'3.1.3.dev1212'

The git sha only found its way into the version number (e.g, 'v3.1.2-1357-g0307c777') in some cases though - when importing Parcels but not installing it using pip. Hence I don't think this is really a regression.

I used xarray's version finding mechanism - note the "9999" version number is to do with pydata/xarray#8200 , which doesn't really apply to us but I thought why not have that as a fallback