do not include plotly.jl twice (JuliaPlots#4684)

* do not include plotly.jl twice

* remove plotly is_subplot_supported

* do not include plotly.jl  twice

src/backends.jl

@@ -534,7 +534,7 @@ _post_imports(::PlotlyBackend) = @eval begin
     const PlotlyKaleido = Main.PlotlyKaleido
     # FIXME: in Plots `2.0`, `plotly` backend should be re-named to `plotlybase`
     # so that we can trigger include on `@require` instead of this
-    include(_path(:plotly))
+    PLOTS_DEFAULT_BACKEND == "plotly" || include(_path(:plotly))
     include(_path(:plotlybase))
 end
 function _initialize_backend(pkg::PlotlyBackend)
@@ -553,7 +553,7 @@ function _initialize_backend(pkg::PlotlyBackend)
         # NOTE: `plotly` is special in the way that it does not require dependencies for displaying a plot
         # as a result, we cannot rely on the `@require` mechanism for loading glue code
         # this is why it must be done here.
-        @eval include(_path(:plotly))
+        PLOTS_DEFAULT_BACKEND == "plotly" || @eval include(_path(:plotly))
     end
 end
 

src/backends/plotly.jl

@@ -1,7 +1,5 @@
 # https://plot.ly/javascript/getting-started
 
-is_subplot_supported(::PlotlyBackend) = true
-# is_string_supported(::PlotlyBackend) = true
 _plotly_framestyle(style::Symbol) =
     if style in (:box, :axes, :zerolines, :grid, :none)
         style