build break in graph_parallel - because of the removed libs/system/build/Jamfile.v2

Problem comes using `Boost 1.89`.

The build error is this:
```
error: Unable to find file or target named
error: 'libs/system/build//boost_system'
error: referred to from project at error:
'../libs/graph_parallel/test'
error: could not resolve project reference
'libs/system/build'
```

I know that boost_system is header-only since `Boost 1.69`.
Also the `libs/system/build/Jamfile.v2` exists in `Boost 1.88`, for compatibility reasons, I presume, but now it has been removed in `Boost 1.89`.

So the reference of
`../../system/build//boost_system`
in
`libs/graph_parallel/test/Jamfile.v2`
**now points to a nonexistent target**.

I found two such invalid references in `Boost 1.89`:
```
libs/graph_parallel/test/Jamfile.v2:12: : requirements ../build//boost_graph_parallel ../../system/build//boost_system ;
libs/graph_parallel/example/Jamfile.v2:9: ../../system/build//boost_system
```

**Summary**: `libs/system/build/Jamfile.v2` has been removed, without checking dependent Jamfiles.
But any automated non-regression build covering `graph_parallel/test` would have caught it instantly :(