mbrtowc: Remove support for OSF/1. * m4/mbrtowc.m4 (gl_MBRTOWC_NULL_ARG2): Remove macro. (gl_FUNC_MBRTOWC): Don't invoke it. Don't define MBRTOWC_NULL_ARG2_BUG. (gl_MBRTOWC_INCOMPLETE_STATE): Remove cross-compilation guess for OSF/1. * lib/mbrtowc.c (rpl_mbrtowc): Ignore MBRTOWC_NULL_ARG2_BUG.