Consider removing `rustc_incremental::persist::fs::dep_graph_path_from`

[pierwill]

This function is only used once, with the current session's directory. Is there a reason to keep it, or should it be replaced by dep_graph_path?

Originally posted by @cjgillot in #90407 (comment)

[pierwill]

@rustbot label +A-incr-comp +C-cleanup

[michaelwoerister]

@pierwill, feel free to make a PR and r? me on it.

[pierwill]

@rustbot claim