Remove Zygote extension requirement for compatibility

Removed the requirement for Zygote extension for backward compatibility.

Author: wouterwln

src/ReactiveMP.jl

@@ -110,7 +110,6 @@ function __init__()
     # For Julia > 1.9 this will be loaded automatically without need in `Requires.jl`
     @static if !isdefined(Base, :get_extension)
         @require Optimisers = "3bd65402-5787-11e9-1adc-39752487f4e2" include("../ext/ReactiveMPOptimisersExt/ReactiveMPOptimisersExt.jl")
-        @require Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f" include("../ext/ReactiveMPZygoteExt/ReactiveMPZygoteExt.jl")
         @require ExponentialFamilyProjection = "17f509fa-9a96-44ba-99b2-1c5f01f0931b" include("../ext/ReactiveMPProjectionExt/ReactiveMPProjectionExt.jl")
     end