ci: drop near-duplicate jobs (#6106)

Remove ten jobs whose coverage is already provided by other jobs:
duplicate ubuntu pypy/graalpy/3.11/3.14t runs, an intel-mac 3.11
smart_holder run, a windows job pinning the default MSVC runtime,
sandwiched clang 14 and gcc 10/C++20 entries, and extra win32 and
win32-debug variants. Also delete the gcc PYBIND11_TEST_OVERRIDE steps,
dead since gcc 12 left the matrix (the windows-2022 job still runs the
same exercise).

Assisted-by: ClaudeCode:claude-fable-5