linux-user/m68k: remove simulator syscall interface

This interface has been introduced in 2005 with the
coldfire implementation (e6e5906b ColdFire target.)
and looks like to do what the linux-user interface already
does with the TRAP exception rather than the ILLEGAL
exception.

This interface has not been maintained since that.
The semi-hosting interface is not removed so coldfire kernel
with semi-hosting is always supported.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20190524162049.806-1-laurent@vivier.eu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>