CrudExpr::{Delete, Insert, Update}: only DbTables are possible, so cleanup InMem path

[Centril]

Description of Changes

• Commit 1 removes a bunch of methods that were unused.
• Commit 2 realizes that all of this Table business is in fact redundant and can be removed as SourceExpr::InMem is impossible and thus we can assume we have a DbTable.
• Commit 3 moves CrudExpr::Update's fn eval_query logic to its own method, like all the other cases, for readability and consistency.
• Commit 4 uses TxMode::unwrap_mut that was introduced in commit 2 in 2 more cases.

API and ABI breaking changes

None

Expected complexity level and risk

1

[mamcx]

LGTM

[mamcx]

NOTE: We need to recover the cases for full CRUD operations on MemTable when we revisit the plans for the vm