Method deletion ambiguity recomputation fails for Union

[timholy]

#25050 recomputed ambiguities but clearly missed a case:

julia> struct A end

julia> struct B end

julia> foo(a::Union{Nothing, A}) = 1
foo (generic function with 1 method)

julia> foo(a::Union{Nothing, B}) = 2
foo (generic function with 2 methods)

julia> mths = methods(foo)
# 2 methods for generic function "foo":
[1] foo(a::Union{Nothing, A}) in Main at REPL[3]:1
[2] foo(a::Union{Nothing, B}) in Main at REPL[4]:1

julia> Base.delete_method(first(mths))

julia> methods(foo)
# 1 method for generic function "foo":
[1] foo(a::Union{Nothing, B}) in Main at REPL[4]:1

julia> foo(nothing)
ERROR: MethodError: foo(::Nothing) is ambiguous. Candidates:
  foo(a::Union{Nothing, A}) in Main at REPL[3]:1
  foo(a::Union{Nothing, B}) in Main at REPL[4]:1
Possible fix, define
  foo(::Nothing)
Stacktrace:
 [1] top-level scope at none:0

(It works fine if you delete first and then define the B method.)