include/configs: drop default definitions of CONFIG_SYS_PBSIZE

Now that the fallback value of CONFIG_SYS_PBSIZE in
include/config_fallbacks.h has been adjusted, remove its definition
from a large number of board configuration files.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>