Removes the dynamic function definition

[johnedquinn]

Relevant Issues

• Adds support for dynamic dispatch of functions #1267

Description

• I realize I left an unused plan node in my PR of dynamic dispatch of functions. In the PR above, there is both a scalar function call (of static, unresolved, and dynamic) and a scalar function definition (resolved, unresolved, dynamic). There need not be a "definition" of a dynamic function call -- as this wouldn't make much sense. Hence, it was unused.
• This PR removes the node. Since it was unused, no other code needs to be modified.

Other Information

• Updated Unreleased Section in CHANGELOG: NO
• Any backward-incompatible changes? NO
• Any new external dependencies? NO
• Do your changes comply with the Contributing Guidelines
  and Code Style Guidelines? YES

License Information

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

Conformance comparison report

	Base (c23ca8c)	e750694	+/-
% Passing	92.33%	92.33%	0.00%
✅ Passing	5372	5372	0
❌ Failing	446	446	0
🔶 Ignored	0	0	0
Total Tests	5818	5818	0

Number passing in both: 5372

Number failing in both: 446

Number passing in Base (c23ca8c) but now fail: 0

Number failing in Base (c23ca8c) but now pass: 0

[rchowell]

Do you mind rebasing on partiql-planner branch now?