lib/keymaps: remove lua attr in type's merge fn

[MattSturgeon]

Rather than having each option remove lua in their apply function, we can do this in the submodule type's merge function.

Yes, it's a little annoying that we have to indent the submodule definition in order to override it's merge attr.

EDIT: even more annoying that we get used but not defined errors... drafting for now

error: The option `keymaps."[definition 1-entry 1]".lua' was accessed but has no value defined. Try setting the option.

[MattSturgeon]

Testing in the repl:

With lib.modules.mergeDefinitions and/or the type's merge function, using lib.nixvim.keymaps.deprecatedMapOptionSubmodule seems to work, but using options..keymaps.type.nestedTypes.elemType does not.

I don't understand, because these two should be the same thing...

This feels like the same weirdness that we ran into when trying to override types.coercedTo for use in transitionType.