Do not write com.schriftgestaltung.customParameter.GSFont.(Axes|Axis Mappings) to UFO?

It's Designspace info and currently it takes precedence over what is in the Designspace, so if you change e.g. axis names and do ufo2glyphs, you'll get errors like

```
Traceback (most recent call last):
File "C:\...\glyphsLib\cli.py", line 189, in main
return options.func(options)
File "C:\...\glyphsLib\cli.py", line 266, in ufo2glyphs font = glyphsLib.to_glyphs(
File "C:\...\glyphsLib\builder\__init__.py", line 145, in to_glyphs
return builder.font
File "C:\...\glyphsLib\builder\builders.py", line 766, in font
self.to_glyphs_instances()
File "C:\...\glyphsLib\builder\instances.py", line 194, in to_glyphs_instances
user_loc = interp(reverse_mapping, design_loc)
File "C:\...\glyphsLib\builder\axes.py", line 607, in interp
if xa <= x <= xb:
TypeError: '<=' not supported between instances of 'float' and 'NoneType'
```

because the UFO lib key may call an axis `"weight"` when the Designspace calls it `"Weight"`, leading to `None`s going around.

Start with the ufo2glyphs entry point in cli.py and follow the conversion paths named in builder/instances.py and builder/axes.py. Reproduce the failure with mismatched axis names such as “weight” and “Weight”; done means the GSFont axis custom parameters are not written to UFO data and conversion no longer produces None-related errors.

Written by the indexing model from the issue text.