remove the <accessor>_init (e.g. getter_init &c.) "optimisation"

<accessor>_init:

    when the entersub optimisation was introduced, the codebase was
    modified so that every Class::XSAccessor accessor was split into
    an entersub-optimizing version (e.g. getter_init) and a "fast"
    version (e.g. getter) without the entersub-optimizing check/code e.g.:

        entersub-optimizing version:

            if (can_optimize(op)) {
                optimize(op);
            }

            // accessor body
            if (items > 1) { ... }

        "fast" version:

            // accessor body
            if (items > 1) { ... }

    profiling shows that this split doesn't provide a speedup and
    may in fact marginally slow things down. removing this
    "optimisation" allows the code and (internal) documentation
    to be simplified.

t/08hash_entersub.t:

    this test relies on the output (via "warn") of the internal (test-only)
    "test" accessor. it used to be almost impossible to modify since, as
    well as matching messages, it also matched line numbers
    i.e. adding/removing a line broke everything. it has been
    rewritten to match the expected output in a less fragile,
    more readable way