bsc/osmo-bsc.cfg: remove legacy 'rtp-base' config

osmo-bsc does not use an rtp-base for years, ever since we're using an MGW.

Related: Id14fa3066ca5d472a817593074a6222f159168a8
Change-Id: Ia4866fc603877b2ab8ce16683e9f6145764117c7
1 file changed