tests: net: all: Remove redundant/misspelled NET_RPL assignments

CONFIG_NET_RPL_MC_ETX was misspelled as "..._EXT".

CONFIG_NET_RPL_MC_{NONE,EXT,ENERGY} are choice symbols. Setting a choice
symbol to 'y' implies that none of the other choice symbols are
selected, so remove the 'n' assignments and keep just the
CONFIG_NET_RPL_MC_ENERGY=y assignment.

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