pmc405de and zeus: remove the env_ptr externs

The env_ptr is already declared in environment.h, so there is no need to
redeclare in board files (especially after including the environment.h)
Remove those declarations.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Matthias Fuchs <matthias.fuchs@esd.eu>