net: lib: config: kconfig: Remove unused NET_CONFIG_SNTP_INIT_PRIO sym

Added in commit 106a0f73 ("net: lib: config: Add SYS_INIT handler to
set clock from SNTP"), never used.

Found with a script.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>