chore(core): remove unused diagram module

[NoahKusaba]

Which issue does this PR close?

Closes #.

Rationale for this change

ballista_core::diagram has no callers anywhere in the repository. It was superseded by
scheduler/src/state/execution_graph_dot.rs and the CLI's job DOT popup, but stayed
publicly exported and carried two #[allow(deprecated)] attributes to keep a deprecated
CoalesceBatchesExec import compiling.

What changes are included in this PR?

• Deleted ballista/core/src/diagram.rs (145 lines).
• Removed the pub mod diagram; declaration from ballista/core/src/lib.rs.

Are there any user-facing changes?

Yes. ballista_core::diagram is removed from the public API of ballista-core. It has no
in-tree users, but this is a breaking change for any downstream consumer — may warrant the
api change label.