Methods Fn(Mut,Once)::call(mut,once) are gated with two feature gates, remove one of them
          #34802
        
          
      
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Methods
Fn::call,FnMut::call_mutandFnOnce::call_onceare gated with usual library featurefn_traitsand also hardcoded in the compiler and gated once more with featureunboxed_closuresThis patch removes the
unboxed_closuresfeature gate from these methods (unboxed_closuresis still used for other things though), now they are gated only withfn_traits.All unnecessary
#![feature(unboxed_closures)]s are removed, many of them are old and were already unnecessary before the change this PR does.