Remove vestigal datafusion-docs module compilation

[alamb]

Which issue does this PR close?

Part of #7013

Rationale for this change

The datafusion-docs module was added in #7952 by @andygrove before we had the ability to test the inline doc examples

However, we can now directly test inline documentation using doctest,

datafusion/datafusion/core/src/lib.rs

Lines 618 to 643 in 37e54ee

	// Instructions for Documentation Examples
	//
	// The following commands test the examples from the user guide as part of
	// `cargo test --doc`
	//
	// # Adding new tests:
	//
	// Simply add code like this to your .md file and ensure your md file is
	// included in the lists below.
	//
	// ```rust
	// <code here will be tested>
	// ```
	//
	// Note that sometimes it helps to author the doctest as a standalone program
	// first, and then copy it into the user guide.
	//
	// # Debugging Test Failures
	//
	// Unfortunately, the line numbers reported by doctest do not correspond to the
	// line numbers of in the .md files. Thus, if a doctest fails, use the name of
	// the test to find the relevant file in the list below, and then find the
	// example in that file to fix.
	//
	// For example, if `user_guide_expressions(line 123)` fails,
	// go to `docs/source/user-guide/expressions.md` to find the relevant problem.

The existence of these old examples now causes confusion (e.g. #12026 from @jc4x4)

Changes:

1. Remove datafusion-docs

Note I will work with @jc4x4 tp add the relevant examples as part of #12026.

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[edmondop]

@alamb relates to #10768

[alamb]

🚀