The LCHAN and BTS filter contexts are actually never used, so drop them until
someone adds them properly.
For now use only LOGGING_{FILTER,CTX}_VLR_SUBSCR. Some of these will change to
_BSC_SUBSCR once struct bsc_subscriber is introduced, and later on, struct
gsm_subscriber will be replaced by vlr_subscriber so that the names will match.
Depends: libosmocore change-id I5c343630020f4b108099696fd96c2111614c8067
Change-Id: Ifa82f6a461ad4c0eeddb8a38fb3833460432d16b