Throw OverflowError on copysign(typemin(Int)//1, 1) (JuliaLang#53395)

The default `copysign(x::Real, y::Real)` in `number.jl` works, so the
incorrect method in `rational.jl` isn't needed.

Here is a benchmark of the new version.
```julia
using BenchmarkTools
function foo!(c,a,b) 
    c .= copysign.(a, b)
    nothing
end
N = 1000
@Btime foo!(c,a,b) setup=(c=zeros(Rational{Int},N); a=rand(Int,N).//rand(Int,N); b=fill(-1,N))
```
On master: 406.215 ns (0 allocations: 0 bytes)
On this PR: 869.327 ns (0 allocations: 0 bytes)

base/rational.jl

@@ -315,8 +315,6 @@ denominator(x::Rational) = x.den
 
 sign(x::Rational) = oftype(x, sign(x.num))
 signbit(x::Rational) = signbit(x.num)
-copysign(x::Rational, y::Real) = unsafe_rational(copysign(x.num, y), x.den)
-copysign(x::Rational, y::Rational) = unsafe_rational(copysign(x.num, y.num), x.den)
 
 abs(x::Rational) = unsafe_rational(checked_abs(x.num), x.den)
 

test/numbers.jl

@@ -679,6 +679,9 @@ end
     @test copysign(big(-1), 0x02) == 1
     @test copysign(big(-1.0), 0x02) == 1.0
     @test copysign(-1//2, 0x01) == 1//2
+
+    # Verify overflow is checked with rational
+    @test_throws OverflowError copysign(typemin(Int)//1, 1)
 end
 
 @testset "isnan/isinf/isfinite" begin