Remove legacy SQL code

[joshua-spacetime]

Description of Changes

This patch removes the dead legacy SQL query engine and the remaining code that only existed to support it.

Removed:

• Old SQL compiler/type-checker and VM-based execution path in spacetimedb-core
• spacetimedb-vm crate
• Dead vm specific error variants and compatibility code
• Obsolete tests, benchmarks, and config paths that still referenced the legacy engine

Small pieces still used by the current engine were moved to their proper homes instead of keeping the vm crate around. In particular, the RelValue was moved to spacetimedb-execution.

The sqltest crate was also updated to use the current engine. Notably though these tests are not run in CI, however I've kept them around as they may be beneficial as we look to expand our SQL support in the future.

API and ABI breaking changes

None

Expected complexity level and risk

1

Testing

None