ppp: dead code cleanup in Kconfig

ppp: dead code cleanup in Kconfig

There is already an 'if PPP' condition wrapping several config options
e.g. PPP_MPPE and PPPOE, making the 'depends on PPP' statement for each of
these a duplicate dependency (dead code).

I propose leaving the outer 'if PPP...endif' and removing the individual
'depends on PPP' statement from each option.

This dead code was found by kconfirm, a static analysis tool for Kconfig.

Signed-off-by: Julian Braha <julianbraha@gmail.com>
Reviewed-by: Qingfang Deng <dqfext@gmail.com>
Link: https://patch.msgid.link/20260330213258.13982-1-julianbraha@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>