remove Base and Core from ambiguities test

Reporting ambiguities that cannot be fixed by the user since they are
not part of the testtarget can be harmful to the user experience since
they will never be useful, since at least JuliaLang/julia#36962.

Fixes #77

src/Aqua.jl

@@ -29,7 +29,7 @@ include("persistent_tasks.jl")
 
 Run the following tests:
 
-* [`test_ambiguities([testtarget, Base, Core])`](@ref test_ambiguities)
+* [`test_ambiguities([testtarget])`](@ref test_ambiguities)
 * [`test_unbound_args(testtarget)`](@ref test_unbound_args)
 * [`test_undefined_exports(testtarget)`](@ref test_undefined_exports)
 * [`test_project_extras(testtarget)`](@ref test_project_extras)
@@ -66,7 +66,7 @@ function test_all(
 )
     @testset "Method ambiguity" begin
         if ambiguities !== false
-            test_ambiguities([testtarget, Base, Core]; askwargs(ambiguities)...)
+            test_ambiguities([testtarget]; askwargs(ambiguities)...)
         end
     end
     @testset "Unbound type parameters" begin