Drop dead code regarding decomissioned Indexers

for decomissioned indexer in ctags [1], language [2]:

• remove code source in swh.indexer

• remove related unused api endpoints in swh.indexer.storage - [x] remove systemd service puppet manifest (already cleaned up)

• [1] !355 (closed)

• [2] !356 (closed)

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Migrated from T4409 (view on Phabricator)