GCC 8 Fix, v27r1 branch (2019.07.09.)

Removed the line that was supposed to add the "right compiler" to the Gaudi environment. Since that extension of the PATH environment variable was only not breaking our 21.X builds by a miracle. Just look at for instance:

/cvmfs/atlas.cern.ch/repo/sw/software/21.2/GAUDI/21.2.81/InstallArea/x86_64-slc6-gcc62-opt/Gaudi.xenv

It has the following in it (amongst a lot of other stuff 😛):

<env:prepend variable="PATH">/bin</env:prepend>

Which is/was coming from this line that I'm proposing to remove now. Since in the current setup ROOT's genreflex is/was always making use of the C++ header files provided by the system's compiler. (Coming from /bin.) But when switching to GCC 8, with which ROOT uses C++17, that doesn't work anymore. And I was getting all sorts of dictionary generation issues during my Gaudi build for the x86_64-centos7-gcc8-opt platform.

Note that in ATLAS builds the correct compiler is always in the runtime environment of the build, Gaudi doesn't need to do anything to put it there.

@leggett, please have a look, and if you agree with the change, make a new tag for me. 😉