samples/net: Reducing prj*.conf files by relevantly removing 6Lo option

If NET_L2_IEEE802154 and NET_IPV6 are enabled:
- NET_6LO is enabled
- NET_L2_IEEE802154_FRAGMENT is enabled

Thus removing setting this config options to yes where applicable.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>