axisregistry: Can we remove spaces from Weight fallback names?

[m4rc1e]

@nathan-williams is it possible to remove spaces from the Weight fallback names so "Semi Bold" becomes "SemiBold etc?

This will match our font style names, https://github.com/googlefonts/gftools/blob/master/Lib/gftools/util/google_fonts.py#L85-L95

[davelab6]

Since the axis' values are not changing I hope this is possible

[nathan-williams]

I think we should preserve the spaces so that we can use the fallback names to generate labels. It's easier to remove the spaces than add them back in.