Do not specify rollback_index_location argument in vbmeta image generation

To match the behavior in make. The module property
rollback_index_location is still needed as it is used to generate
chain_partition arguments.

Test: verify that rollback_index_location argument is never set in make vbmeta partitions
Test: diff soong vs make vbmeta_system partitions
Bug: 390204058
Change-Id: I434388a5a292086746dac58e41d034cdbcdb42a3