Remove outputs from roxygen examples

[IndrajeetPatil]

In pkgdown, this output is right below the code.
In RStudio IDE, users can just click on "Run examples", and it will show the output.
Everywhere else, the users can execute the code and see the outputs. I don't think we need to embed the outputs here.

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Another issue is that styler keeps restyling this code, which needs to be reverted:

styler::style_text('get_r_string(expr_as_xml4.0, "expr[2]") # "a\\b"')
#> get_r_string(expr_as_xml4.0, "expr[2]") # "a\b"