fix: remove dead test variables

[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

Remove dead -Wunused-but-set-variable test variables

Clears ~19 clang -Wunused-but-set-variable warnings per CI job. All changes are
test-only with no behavioral effect: each removed variable was written but never read.

• test/graph.cpp (E): An edge counter (++E on add, --E on remove) introduced with the original graph test file (Jeremy Siek, 2000) that was never read in the entire history of the file. The actual edge-count invariant is already verified via count_edges(g), so E was redundant scaffolding from day one. Removed the declaration and its three writes.

• test/property_iter.cpp (E): The same write-only edge-counter idiom, never read. This test exercises property iterators, so the counter was never relevant. Removed the declaration and its single increment.

• test/csr_graph_test.cpp (edge_count block): Originally (Douglas Gregor, 2005) this counted a vertex's out-edges to verify the CSR edge() and edge_range() functions. Those functions were deliberately removed in 2009 (Jeremiah Willcock) as unsupportable under the CSR model, along with the assertions that used the count, but the counting loop was left behind. Removed the whole orphaned block (also clearing a latent unused oei2_end and avoiding cascading -Wunused-variable/-Wempty-body warnings that deleting only the variable would cause).

These can't be repurposed into assertions: the E counters have no readable invariant (the real edge-count check already lives in count_edges(g), and num_edges == E would be wrong for multigraph configs), and edge_count only ever backed assertions that were deliberately removed in 2009 with the edge()/edge_range() functions.

Motivation

See #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 27948199982 vs develop run 27866501982 (505d1ca260).

Job	Baseline	After	Delta
macos (clang, 14)	46	27	-19
macos (clang, 17)	41	22	-19
macos (clang, 20)	41	22	-19
ubuntu (clang-19, 14)	46	27	-19
ubuntu (clang-19, 17)	46	27	-19
ubuntu (clang-19, 20)	46	27	-19
ubuntu (clang-19, 23)	46	27	-19
ubuntu (gcc-14, 14)	28	28	0
ubuntu (gcc-14, 17)	28	28	0
ubuntu (gcc-14, 20)	28	28	0
ubuntu (gcc-14, 23)	28	28	0
windows_msvc_14_3 (msvc-14.3)	974	974	0