Remove const qualification in compile-time checks

[jackoalan]

A regression appeared in dd8cc8b for compile-time checking of const references.

Prior to dd8cc8b, all callers of make_args_checked had the equivalent of remove_const_t and remove_reference_t via parameter type deduction. Now only remove_reference_t is present which means custom type formatter specializations will not be selected unless they have been explicitly specialized with a const type. I am assuming that this is not the intended behavior and have added remove_const_t in addition to remove_reference_t.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks!