[ci] remove unnecessary workarounds in R-package macOS jobs

[jameslamb]

In #6266, I'm working on making this project's shell scripts stricter, so they exit with a non-0 exit code the first time anything goes wrong (e.g. some variable they require isn't set or somme command they run fails).

This revealed that 2 workarounds in .ci/test_r_package.sh that are no longer necessary:

• symlinking R to /usr/local/bin/R (added in [ci] upgrade to R 4.1.2 in CI, change approach to macOS-latest R CI builds (fixes #4988) #4989)
• ensuring the R's libomp.dylib and Homebrew's libomp.dylib don't conflict with each other (added in [R-package][ci] added CI stage for R package (fixes #2335, fixes #2569) #2530)

Those have both been unnecessary and not working for a while, as can be seen by messages like this in CI:

ln: /Library/Frameworks/R.framework/Versions/4.3/Resources/lib/libomp.dylib: No such file or directory

(link to recent build from master)

This removes those workarounds, simplifying the testing setup a bit.