Check coverage of chardata handling code + purge unreachable

[pp-mo]

Additional to #6898

Perform code coverage analysis against all iris tests
Untouched code is then either redundant or untested

Various parts of the code used to handle char<->string encoding, which is now done by "encoded datasets"
Notably:

• fileformats.cf
• fileformats._nc_load_rules -- mostly in helpers
• fileformats.netcdf -- both loader and saver

Both load + save should now support the use of either string-arrays or char-arrays.
And all those behaviours should be tested.
But some obsolete code may have slipped though the net