Remove RuntimeError handling obsolete since mock 3.0.0 (#147)

[Rishithar2207]

Closes #147.

The RuntimeError: stop called on unstarted patcher handling in unwrap_assert_methods was added for #137, when calling stop() on an already-stopped patcher raised. That raise was removed from CPython in mock 3.0.0 (bpo-36366), so the branch is unreachable on any currently supported version.

Verification:

• pytest-mock requires Python >=3.10; the raise was removed in mock 3.0.0 / CPython 3.8.
• The string "stop called on unstarted patcher" does not appear anywhere in unittest/mock.py on 3.12.
• Calling patcher.stop() twice on 3.12 is a silent no-op rather than raising.

No behaviour change for users. Local test results are unchanged from main (the four failing introspection tests fail identically before and after this change, unrelated to it).