npcx: scfg: pinmux: cleanup unused functions and macro

Clean up unused pinmux function 'npcx_pinctrl_mux_configure()' and
related 'NPCX_DT_IO_ALT*' macro functions.

Please refer https://issuetracker.google.com/234861079

 for more detail.

Signed-off-by: Mulin Chao <mlchao@nuvoton.com>