db, view: remove duplicate entries from pending endpoints
db, view: remove duplicate entries from pending endpoints

When generating view updates, an endpoint can appear both
as a primary paired endpoint for the view update, and as a pending
endpoint (due to range movements). In order not to generate
the same update twice for the same endpoint, the paired endpoint
is removed from the list of pending endpoints if present.

Fixes #5459
Tests: unit(dev),
       dtest(TestMaterializedViews.add_dc_during_mv_insert_test)

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla