Remove obsolete sp exit flow from sp intro workflow page

[deepcrazy]

Description

• removed obsolete sp exit flow from sp intro workflow page (https://docs.bnbchain.org/greenfield-docs/docs/guide/storage-provider/introduction/workflow/#sp-exit)

Rationale

• As there are new process available for sp exit over here: https://docs.bnbchain.org/greenfield-docs/docs/guide/core-concept/storage-provider-lifecycle/#exit and https://docs.bnbchain.org/greenfield-docs/docs/guide/storage-provider/run-book/exit-SP-network/