mesh: Fix typo when deleting app key in CDB
mesh: Fix typo when deleting app key in CDB

This commit sets `net_idx` to `BT_MESH_KEY_UNUSED` when deleting app key in
cdb.

this is port of d593af30db1c9164db35ccc28862b06718440cad

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble