fix(api-public): remove orphan import_export actors module

The actors/import_export.rs file was added in 34422ea but its
dependencies (rivet-envoy-protocol, serde_bare, rivet-api-types
import_export module, pegboard::actor_sqlite_v2, pub visibility of
list_inner) were never ported from the unmerged import/export-actors
branch. Result: rivet-api-public fails to compile on main, breaking
all engine Docker builds and Rust CI.

Drop the file and its mod.rs/router.rs wiring until the supporting
pieces land.

Author: NathanFlurry