Remove unused/broken UI and addin functionality

[juliasilge]

This PR removes/deprecates functions for viewing legacy API pins in the RStudio Viewer pane. Some of these didn't work anymore:

library(pins)
ui_addin_pin_find()
#> Error in `shiny::shinyAppDir()` at pins-r/R/ui_addin.R:10:2:
#> ! No Shiny application exists at the path ""

#> Backtrace:
#>     ▆
#>  1. └─pins::ui_addin_pin_find()
#>  2.   └─shiny::shinyAppDir(system.file("rstudio/shinyaddin", package = "pins")) at pins-r/R/ui_addin.R:10:2
#>  3.     └─rlang::abort(...)

^{Created on 2022-11-18 with reprex v2.0.2}

library(pins)
ui_connection_create()
#> Error in `shiny::shinyAppDir()` at pins-r/R/ui_connection.R:10:2:
#> ! No Shiny application exists at the path ""

#> Backtrace:
#>     ▆
#>  1. └─pins::ui_connection_create()
#>  2.   └─shiny::shinyAppDir(system.file("rstudio/shinycon", package = "pins")) at pins-r/R/ui_connection.R:10:2
#>  3.     └─rlang::abort(...)

^{Created on 2022-11-18 with reprex v2.0.2}

Some of this may still work but would not be a part of typical production or scheduled jobs because these are intended for interactive use only.

[juliasilge]

This will let us move forward with #678 more easily.

[juliasilge]

I deprecated this because I think it is still somewhat usable in the current release, but it has been @keywords internal and documented as "no longer used" for over a year so we probably could remove it altogether. Again, this is not something that would typically be used in production or scheduled jobs.

[juliasilge]

As far as I can tell from my interactive work and the automated tests, functionality like read/write/list/etc for the legacy API all are unaffected by these changes.

[github-actions]

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