Revert "tweak after #492: capture.output() does withVisible() and print() if necessary"

This reverts commit 451f950d0dd54ad57ec5b9b5ab465a2ac6d29a9b.

Too-aggressive capture.output was causing browser() not to work in
renderPlot().
Merge pull request #526 from rstudio/bugfix/plot-captures-output

Revert "tweak after #492: capture.output() does withVisible() and print(...
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