Remove multidimensional indexing of tuples

Previously a method for tuples existed which permited multidimensional
indexing. However, the method merely splatted the dimensions and ended
up calling itself, infinitely recursing. This commit removes the method
entirely and adds tests to ensure the behavior doesn't return.

base/multidimensional.jl

@@ -384,7 +384,6 @@ uncolon(inds::Tuple,      I::Tuple{Colon, Vararg{CI0}}) = Slice(OneTo(trailingsi
 
 ### From abstractarray.jl: Internal multidimensional indexing definitions ###
 getindex(x::Number, i::CartesianIndex{0}) = x
-getindex(t::Tuple, I...) = getindex(t, IteratorsMD.flatten(I)...)
 
 # These are not defined on directly on getindex to avoid
 # ambiguities for AbstractArray subtypes. See the note in abstractarray.jl

test/tuple.jl

@@ -231,3 +231,8 @@ end
 @test Tuple{Int,Vararg{Any}}(Float64[1,2,3]) === (1, 2.0, 3.0)
 @test Tuple(ones(5)) === (1.0,1.0,1.0,1.0,1.0)
 @test_throws MethodError convert(Tuple, ones(5))
+
+@testset "Multidimensional indexing (issue #20453)" begin
+    @test_throws MethodError (1,)[]
+    @test_throws MethodError (1,1,1)[1,1]
+end