Remove outdated `rewrite_expr.rs` example

[alamb]

Draft while we fix the other examples

Which issue does this PR close?

Part of https://github.com/apache/datafusion/issues/10855

Rationale for this change

rewrite_expr.rs has three distinct examples within it it. These now all have their own examples (done in the following PRs)

1. Using DataFusion LogicalPlanning without execution: https://github.com/apache/datafusion/pull/11088
2. Implementing an AnalyzerRule: https://github.com/apache/datafusion/pull/11089
3. Implementing an OptimizerRule: https://github.com/apache/datafusion/pull/11087

What changes are included in this PR?

1. Remove (now) redundant example

Are these changes tested?

Are there any user-facing changes?