GH-49380: [R] Remove hidden CI test chunks from setup.Rmd to fix r-de… (#49381)

### Rationale for this change

The `r-devdocs` crossbow CI job fails during the gap between the C++
release (published to apt) and the R package release (CRAN). Two hidden
bash chunks in `setup.Rmd` were silently installing the released
`libarrow-dev` from apt during CI, overwriting the version built from
the git checkout. Since the R package (`arrow.so`) was compiled against
the newer git-built `libarrow`, loading it against the older apt version
caused an undefined symbol ABI mismatch error.

### What changes are included in this PR?

Removes the hidden CI test chunks from
`r/vignettes/developers/setup.Rmd`:

- Removed hidden macOS chunk that ran `brew install apache-arrow`
- Removed hidden Ubuntu chunk that ran `apt install libarrow-dev` (root
cause of the ABI mismatch)
- Converted the `MAKEFLAGS="LDFLAGS="` example from an executable knitr
chunk to a plain `bash` code block preserves the documentation example
without executing it in CI

### Are these changes tested?

No, the fix removes the broken test logic from the vignette. As noted by
@thisisnic, this type of testing should live in proper CI
infrastructure, not embedded as hidden code in documentation.

### Are there any user-facing changes?

No. The rendered documentation is unchanged the `MAKEFLAGS="LDFLAGS="`
example remains visible in the "Multiple versions of libarrow"
troubleshooting section.

* GitHub Issue: #49380

---------

Co-authored-by: vanshaj2023 <vanshaj2023@users.noreply.github.com>
Co-authored-by: Jonathan Keane <jkeane@gmail.com>

Author: vanshaj2023

r/vignettes/developers/setup.Rmd

@@ -350,7 +350,7 @@ sudo apt update
 sudo apt install -y -V libarrow-dev
 ```
 
-```{bash, save=run & !sys_install & macos}
+```{bash, save=run & !sys_install}
 MAKEFLAGS="LDFLAGS=" R CMD INSTALL .
 ```