docs/sources/configuration: Remove obsoleted "mpls-label-range"
docs/sources/configuration: Remove obsoleted "mpls-label-range"

see be9918a163a20a7a885dc9206a733d983ee76b3a

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>

osrg/gobgp,a16/gobgp,tamihiro/gobgp,j-kato/gobgp,fujita/gobgp,a16/gobgp,fujita/gobgp,fujita/gobgp,j-kato/gobgp,a16/gobgp,j-kato/gobgp,a16/gobgp,osrg/gobgp,fujita/gobgp,Dipsingh/gobgp,a16/gobgp,osrg/gobgp,a16/gobgp,tamihiro/gobgp,osrg/gobgp,Dipsingh/gobgp,Dipsingh/gobgp,a16/gobgp,tamihiro/gobgp,tamihiro/gobgp