Kconfig: Remove redundant variable sets

In a few places we have Kconfig entries that set SPL_LDSCRIPT to what is
the default value anyways.  Drop these.

Cc: Michal Simek <monstr@monstr.eu>
Cc: Rick Chen <rick@andestech.com>
Cc: Philippe Reynes <tremyfr@yahoo.fr>
Cc: Eric Jarrige <eric.jarrige@armadeus.org>
Signed-off-by: Tom Rini <trini@konsulko.com>