Internal: remove bwc leftover

This caused some test failures as the readFrom method was not in sync anymore with the writeTo
Internal: remove some more bwc code

Some more code that was left for backwards compatibility can now be removed on master since 2.0 will require a full cluster restart.

Closes #8778
Internal: remove optional original indices

Original indices are optional in ShardDeleteByQueryRequest only for backwards compatibility, see #7406. We can remove this in master since 2.0 will require a full cluster restart.

Closes #8777