[FEATURE] Remove Engine composition

[goodwanghan]

We are able to combine an execution engine with a sql engine for example

native+duckdb

would use native execution engine + duckdb sql engine.

But now

1. different sql engines can be used inside any FugueWorkflow (CONNECT in Fugue SQL)
2. ExecutionEngine now consists of multiple components: SQLEngine, MapEngine, etc. The current expression using + only works to overwrite SQLEngine which no longer makes sense
3. This feature is rarely used

So in order to keep things simple, we should remove this feature. We may add back when there is a better design.