In commit 104a6a7 support was added
for Thumb2.  It added do_execute() as a way to provide arch dependent
calling veneers for use in "go" and thumb2_execute() as the thumb2 to
arm veneer.

But thumb2_execute() isn't necessary as gcc generates a proper calling
sequence from a standard function pointer call.  Thumb2 barebox is
compiled with the AAPCS ABI which requires this.

It also had a bug and didn't pass the arguments properly, but code
execute via "go" rarely uses arguments so this wasn't very noticeable.

Since thumb2 was always the only user of do_execute(), go ahead and
delete that too.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>