remove exploded_tag_ssim as it is no longer used

[ndushay]

Why was this change made? 🤔

• Getting rid of cruft
• freeing up space on sul-solr boxes (exploded tags are stored and are large fields)

Closes #1025 Part of #1047

How was this change tested? 🤨

• github search on sul-dlss repos to make sure tag isn't used https://github.com/search?q=org%3Asul-dlss+exploded_tag_ssim&type=code
• CI
• sul-dlss/infrastructure-integration-test/pull/650

⚡ ⚠ If this change has cross service impact, run integration tests that exercise indexing (e.g. searches in Argo for newly created/updated items, access_indexing_spec) and/or test in [stage|qa] environment, in addition to specs. ⚡