Compute dof level kernel PSyKAl-lite routine

There is currently a PSyKAl-lite routine `invoke_compute_dof_level_kernel` in LFRic apps `science/gungho/source/psy/psykal_lite_mod`, which is called by `science/gungho/source/algorithm/diagnostics/diagnostic_alg_mod.x90`.

Although the PSyclone issue was not listed in the code, I believe that this kernel is related to PSyclone's ability to have user-defined kernels which act on dofs (using the information provided here https://code.metoffice.gov.uk/trac/lfric/wiki/PSycloneTool/PSyKAlLite). The original PSyclone ticket for this behaviour was [PSyclone #1351](https://github.com/stfc/PSyclone/issues/1351), which was completed via [PSyclone #2711](https://github.com/stfc/PSyclone/issues/2711).

As I've understood it, this may now be able to be removed.

Inspect science/gungho/source/psy/psykal_lite_mod and its call from science/gungho/source/algorithm/diagnostics/diagnostic_alg_mod.x90. Read the linked PSyclone issues and verify whether the routine is still needed with the completed dof-kernel support. Done means removing obsolete code without breaking the relevant LFRic build or diagnostics path.

Written by the indexing model from the issue text.