BREAKING CHANGE: drop support for X_DIM and friends, remove deprecated backend functions

[romanc]

Description

This PR removes things deprecated with the February release. In particular, it

• drops support for X_DIM and friends (use I_DIM and friends)
• removes is_gpu_backend() and backend_is_fortran_aligned() from gt4py_utils since those attributes can and should now be queried on the Backend object.

How has this been tested?

All good as long as CI is green.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas: N/A
• I have made corresponding changes to the documentation (e.g. add new modules to docs/docstrings/): N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules: N/A
• New check tests, if applicable, are included: N/A

[oelbert]

👍