a/bindings: Remove xrt_[input|output]_name_string functions
These were buggy anyways as inputs and outputs not referred to by the
json wouldn't be listed, for instance all the PS Sense inputs. Replace
usage with u_str_xrt_input_name instead.
Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2633>
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