rstudio: Fix recompilation issue with custom package set

This attempts to fix the issue described at
https://github.com/NixOS/nixpkgs/pull/22219#issuecomment-291801133.
Any change to the custom packages passed to RStudio causes this to
completely rebuild RStudio, which is completely unnecessary and also a
bit of a hindrance as it's a fairly slow build.

This rolls back most of that old PR, and instead implements something
more like rWrapper.  Existing configurations with the old useRPackages
will break.

pkgs/applications/editors/rstudio/default.nix

@@ -1,13 +1,5 @@
 { stdenv, fetchurl, makeDesktopItem, cmake, boost163, zlib, openssl,
-R, qt5, libuuid, hunspellDicts, unzip, ant, jdk, gnumake, makeWrapper, pandoc,
-# If you have set up an R wrapper with other packages by following
-# something like https://nixos.org/nixpkgs/manual/#r-packages, RStudio
-# by default not be able to access any of those R packages. In order
-# to do this, override the argument "R" here with your respective R
-# wrapper, and set "useRPackages" to true.  This will add the
-# environment variable R_PROFILE_USER to the RStudio wrapper, pointing
-# to an R script which will allow R to use these packages.
-useRPackages ? false
+R, qt5, libuuid, hunspellDicts, unzip, ant, jdk, gnumake, makeWrapper, pandoc
 }:
 
 let
@@ -111,14 +103,8 @@ stdenv.mkDerivation rec {
     mimeType = "text/x-r-source;text/x-r;text/x-R;text/x-r-doc;text/x-r-sweave;text/x-r-markdown;text/x-r-html;text/x-r-presentation;application/x-r-data;application/x-r-project;text/x-r-history;text/x-r-profile;text/x-tex;text/x-markdown;text/html;text/css;text/javascript;text/x-chdr;text/x-csrc;text/x-c++hdr;text/x-c++src;";
   };
 
-  postInstall = let rProfile =
-    # RStudio seems to bypass the environment variables that the R
-    # wrapper already applies, and so this sets R_PROFILE_USER to
-    # again make those R packages accessible:
-    if useRPackages
-    then "--set R_PROFILE_USER ${R}/${R.passthru.fixLibsR}" else "";
-    in ''
-      wrapProgram $out/bin/rstudio --suffix PATH : ${gnumake}/bin ${rProfile}
+  postInstall = ''
+      wrapProgram $out/bin/rstudio --suffix PATH : ${gnumake}/bin
       mkdir $out/share
       cp -r ${desktopItem}/share/applications $out/share
       mkdir $out/share/icons