remove extraneous function included in #21858 (#59109)

Removes an apparently extraneous function accidentally included in
#21858, as noted in
https://github.com/JuliaLang/julia/pull/21858/files#r2233250284.

Author: stevengj

base/complex.jl

@@ -1142,10 +1142,6 @@ function complex(A::AbstractArray{T}) where T
     convert(AbstractArray{typeof(complex(zero(T)))}, A)
 end
 
-## Promotion to complex ##
-
-_default_type(T::Type{Complex}) = Complex{Int}
-
 ## Machine epsilon for complex ##
 
 eps(z::Complex{<:AbstractFloat}) = hypot(eps(real(z)), eps(imag(z)))