[R-package] [cmake] simplify CMake-based R builds

[jameslamb]

Proposes some small simplifications to install.libs.R, the R script that runs cmake --build in CMake-based builds of the R package.

• removes references to .Internal(internalsID())
  • this is no longer necessary now that we link against R.so directly: [R-package] Factor out custom R interface to lib_lightgbm #3016
  • ref: https://github.com/wch/r-source/blob/0986e80068addc5c884d06580412ff2331436814/src/include/Defn.h#L106-L108
• removes unnecessary assignments in setting up CMake defines that are always, unconditionally used

[StrikerRUS]

LGTM, thanks!