Remove workaround for `xconv` in `fix_long_name`

[atteggiani]

Overview

Based on this comment, the xconv long name fix is not too relevant anymore and can be safely removed.

Solution

Remove xconv fix and related variables/tests from um2nc.

[MartinDix]

Just as a demonstration of standard_name length

from iris import std_names
for n in std_names.STD_NAMES:
    if len(n) > 110:
        print(len(n), n)

Our record holder with a length of 166 tendency_of_atmosphere_mass_content_of_particulate_organic_matter_dry_aerosol_particles_expressed_as_carbon_due_to_emission_from_residential_and_commercial_combustion

[atteggiani]

WOW.
Can't that standard name length be reduced? 😅