net: sntp: remove CONFIG_TIMESTAMP constraint

CONFIG_TIMESTAMP is not related to the RTC drivers. It does not make any
sense to let the updating of the RTC by the sntp command depend on it.

Drop the CONFIG_TIMESTAMP checks.

Furthermore function dm_rtc_set() is enabled by CONFIG_DM_RTC. There is no
reason to require CONFIG_CMD_DATE when using a driver model RTC. The UEFI
sub-system can consume the RTC functions even if there is not date command.

Only check CONFIG_CMD_DATE when using a non-driver model RTC.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>