Remove temp index from software licenses cleanup migration

What does this MR do and why?

This MR removes a temporary index introduced to reduce the cleanup migration time in !114158 (merged)

How to set up and validate locally

Before the migration:

1. Run bin/rails dbconsole
2. \d "software_licenses"
3. Check that index is present: "tmp_index_for_software_licenses_spdx_identifier_cleanup" btree (spdx_identifier) WHERE spdx_identifier IS NULL

Migrate and check for removed index:

1. Run bin/rails db:migrate
2. Run bin/rails dbconsole
3. \d "software_licenses"
4. Check that index tmp_index_for_software_licenses_spdx_identifier_cleanup is not listed anymore

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #396528 (closed)