defconfig: sa2150p: remove unused regulator for APC

We are not using the External buck for APC. PMIC
SMPS3 (S3) is used for APC supply.

External buck for APC was only a backup option and
not used on board now hence removed.

Change-Id: I328391d551cae3ccce4f9565f01029d5ce76e034
Signed-off-by: Rishi Gupta <rishgupt@codeaurora.org>