remove system name from function signatures/selectors

[holic]

Instead of {namespace}_{systemName}_{functionName} for world function selectors, just use {namespace}_{functionName}

This has a bunch of benefits:

• more closely matches root systems where the world function doesn't indicate which system the function is in
• more closely matches how namespaced tables ({namespace}_{tableName}) are accessed in downstream tooling
• more ergonomic
• removes the need for us to add validation for system names (to ensure they don't have underscores)