Remove string handling in ArgOrdering subclasses

[hiker]

ATM all ArgOrdering subclasses return the argument as strings. In #1883 support for getting the same information as PSyIR was added, but the old, string-based results are still supported and used everywhere (the PSyIR results are only needed for LFRic driver creation atm).

The string-handling should be replaced with the PSyIR based results.

[arporter]

Related to this, upon construction, an instance of ArgOrdering stores a reference to a SymbolTable. As noted by @sergisiso (#2313 (comment)) this is dangerous as we end up with a double reference which won't work e.g. if the tree is copied.

[rupertford]

Whilst ArgOrdering makes the LFRic world go round, there is a new base class implementation based on LFRic metadata in MetadataToArgumentsRules.py. My idea is to use this to replace ArgOrdering with this and then modify/re-write the subclasses as needed. There is PR #2079 that is that is looking to fully implement the equivalent of kernel_interface as a first step (although it is already used in PSyAD for index mapping).