fix precompilation issue

[lucaferranti]

In the previous PR I had accidentally the same code copy-pasted twice... shame on me!! 🤦‍♂️ 🙊 This gave some precompilation issues as it was redefining the methods. Fixed now

julia> using IntervalContractors
[ Info: Precompiling IntervalContractors [15111844-de3b-5229-b4ba-526f2f385dc9]
WARNING: Method definition mul_rev_to_pair(IntervalArithmetic.Interval{T} where T<:Real, IntervalArithmetic.Interval{T} where T<:Real) in module IntervalContractors at /home/lferrant/.julia/dev/IntervalContractors/src/arithmetic.jl:327 overwritten at /home/lferrant/.julia/dev/IntervalContractors/src/arithmetic.jl:338.
  ** incremental compilation may be fatally broken for this module **

WARNING: Method definition mul_rev_to_pair(IntervalArithmetic.DecoratedInterval{T}, IntervalArithmetic.DecoratedInterval{T}) where {T} in module IntervalContractors at /home/lferrant/.julia/dev/IntervalContractors/src/arithmetic.jl:329 overwritten at /home/lferrant/.julia/dev/IntervalContractors/src/arithmetic.jl:340.
  ** incremental compilation may be fatally broken for this module **