ARM: SAMSUNG: remove "config S3C_BOOT_WATCHDOG"

All users of S3C_BOOT_WATCHDOG got removed in commit 1ff5f55a
("ARM: samsung: remove unused arch_decomp_wdog() code"). Remove
this symbol too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>