Remove _instantiate from tests

Author: blegat

test/interface.jl

@@ -51,11 +51,6 @@ Base.@irrational theodorus 1.73205080756887729353 sqrt(big(3))
         i_theodorus() = MA.promote_operation(+, Int, typeof(theodorus))
         @test i_theodorus() == Float64
         alloc_test(i_theodorus, 0)
-        # test _instantiate(::Type{S}) where {S<:Irrational} return value
-        @test MA._instantiate(typeof(π)) == π
-        @test MA._instantiate(typeof(MathConstants.catalan)) ==
-              MathConstants.catalan
-        @test MA._instantiate(typeof(theodorus)) == theodorus
     end
 end