Cortex-M3: don't chain "struct arm" commands

Those commands presume support for the "classic" set of CPU
modes (FIQ, supervisor, IRQ, etc) ... which aren't supported
by the ARMv7-M or ARMv6-M architectures.  They also presume
a "struct arm" base type, which this code doesn't use.

We haven't cleaned up the register handling enough to be able
to share any of those "base" methods.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/target/cortex_m3.c

@@ -1930,9 +1930,6 @@ static const struct command_registration cortex_m3_exec_command_handlers[] = {
 	COMMAND_REGISTRATION_DONE
 };
 static const struct command_registration cortex_m3_command_handlers[] = {
-	{
-		.chain = arm_command_handlers,
-	},
 	{
 		.chain = armv7m_command_handlers,
 	},