SupportDefs.h: remove the unichar type

irssi uses the unichar typename with a different type.

Change-Id: I2ee9dcf685f4832dd1b3c7d68f4686d69de50fb7
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3656
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

headers/os/support/SupportDefs.h

@@ -47,7 +47,6 @@ typedef volatile unsigned short	vushort;
 typedef volatile unsigned char	vuchar;
 
 typedef unsigned char			uchar;
-typedef unsigned short			unichar;
 
 /* descriptive types */
 typedef int32					status_t;