arm: am437x: cm-t43: get rid of enable_vtt_regulator()

CM-T43 does not have a vtt regulator. Remove the function that's supposed
to enable it.

Cc: Tom Rini <trini@konsulko.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Reviewed-by: Tom Rini <trini@konsulko.com>