lang: Remove rent_exempt constraint

[acheroncrypto]

Problem

Rent exempt checks are redundant because:

• It is no longer possible to create rent paying accounts(Prevent rent-paying account creation solana-labs/solana#22292)
• Rent collection is being disabled(Add a feature to disable rent collection solana-labs/solana#33945)
• There are no more rent paying accounts remaining(docs: Add Solana Playground quickstart page, and add examples page for anchor contraints and types.  #2703 (comment))

Summary of changes

• Remove rent_exempt constraint
• Remove all tests related to rent exemption
• Remove the ConstraintRentExempt error but keep the error order the same both in lang and ts
• Update rent exemption related docs

[vercel]

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