Remove Base.convert(::Type{Decimal}, ::Real)

The method is unnecessary. We define constructors from `Real`s, which is
enough for the fallback `convert` method to work.

This commit also removes a useless test file.

src/conversion.jl

@@ -24,8 +24,6 @@ function Decimal(x::AbstractFloat)
     return Decimal(s, c, q)
 end
 
-Base.convert(::Type{Decimal}, x::Real) = Decimal(x)
-
 function Base.BigFloat(x::Decimal)
     y = BigFloat(x.c)
     if x.q ≥ 0

test/runtests.jl

@@ -3,18 +3,8 @@ using Test
 
 @testset "Decimals" begin
 
-global d = [
-    Decimal(false, 2, -1)
-    Decimal(false, 1, -1)
-    Decimal(false, 100, -4)
-    Decimal(false, 1512, -2)
-    Decimal(true, 3, -2)
-    Decimal(true, 4, -6)
-]
-
 include("test_arithmetic.jl")
 include("test_bigint.jl")
-include("test_constructor.jl")
 include("test_context.jl")
 include("test_decimal.jl")
 include("test_equals.jl")