fix: silence -Wunused-local-typedef and -Wunused-variable in four examples

[Becheler]

Refs #496

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)
• No

What this PR does

Silence -Wunused-local-typedef and -Wunused-variable in four examples (clears 7 warnings/job on both clang and gcc, example-only, no behavioral change).

• graph-assoc-types.cpp: kept the illustrative associated-type typedefs but marked them with boost::ignore_unused<>() (portable across C++14, not [[maybe_unused]]).
• quick-tour.cpp: removed the dead iter_t typedef (the loop uses auto).
• minimum_degree_ordering.cpp: removed the unused Nrhs variable.
• two_graphs_common_spanning_trees.cpp: added (void) vec; in the placeholder loop body.

Motivation

Refs #496

Testing

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 27949206924 vs develop run 27866501982 (30013adff1).

Job	Baseline	After	Delta
macos (clang, 14)	46	39	-7
macos (clang, 17)	41	34	-7
macos (clang, 20)	41	34	-7
ubuntu (clang-19, 14)	46	39	-7
ubuntu (clang-19, 17)	46	39	-7
ubuntu (clang-19, 20)	46	39	-7
ubuntu (clang-19, 23)	46	39	-7
ubuntu (gcc-14, 14)	28	21	-7
ubuntu (gcc-14, 17)	28	21	-7
ubuntu (gcc-14, 20)	28	21	-7
ubuntu (gcc-14, 23)	28	21	-7
windows_msvc_14_3 (msvc-14.3)	974	971	-3