rr: don't overwrite all of its C and CXX flags

These CMake flags have been present ever since the package was added in
2014. There is no documentation on why these flags should be necessary.
During local testing, rr built just fine without them.

In fact, I stumbled upon this when rr from nixpkgs failed to record
applications that use OpenGL, such as glxinfo or glxgears, whereas a
manually built rr worked just fine. I tracked it down to these flags.

I wasn't able to determine the relevant differences between the two
builds of rr.

pkgs/development/tools/analysis/rr/default.nix

@@ -44,8 +44,6 @@ stdenv.mkDerivation rec {
   ];
   propagatedBuildInputs = [ gdb ]; # needs GDB to replay programs at runtime
   cmakeFlags = [
-    "-DCMAKE_C_FLAGS_RELEASE:STRING="
-    "-DCMAKE_CXX_FLAGS_RELEASE:STRING="
     "-Ddisable32bit=ON"
   ];