Remove getters from pallet-membership (#4840)

As per #3326 , removes pallet::getter macro usage from
pallet-membership. The syntax StorageItem::<T, I>::get() should be used
instead. Also converts some syntax to turbo and reimplements the removed
getters, following #223

cc @muraca

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Co-authored-by: Dónal Murray <donalm@seadanda.dev>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>