Eliminate the possibility of a single render_widget output from keeping a view of prior renders

[cpsievert]

Follow up (in a sense) to #191 and #195. Now that we only close widgets (server-side) when they're created in an output context, it is technically possible to render the same widget repeatedly in a render function, which leads to duplicated views of that widget. For example, clicking the button here will generate duplicate views:

import plotly.express as px
from shiny.express import input, ui

from shinywidgets import as_widget, render_plotly

ui.input_action_button("redraw", "Redraw")

p = px.histogram(
    px.data.tips(),
    x="total_bill",
)
w = as_widget(p)

@render_plotly
def plot():
    input.redraw()
    return w

This PR fixes that issue by resurrecting some cleanup code we had before doing server-side cleanup