scons: no need to set RTT_ROOT as Repository

In SCons manual:

=================
In order to inform the C compiler about the repositories,  SCons will
add appropriate -I flags to the compilation commands for each directory
in the $CPPPATH list. So if we add the current directory to the
construction environment $CPPPATH like so:

    env = Environment(CPPPATH = ['.'])
    env.Program('hello.c')
    Repository('/usr/repository1')

Then re-executing SCons yields:

    % scons -Q
    cc -o hello.o -c -I. -I/usr/repository1 hello.c
    cc -o hello hello.o
=================

The additional include directory is definitely not what we want. Just
remove the additional Repository line.

tools/building.py

@@ -192,8 +192,6 @@ def PrepareBuilding(env, root_directory, has_libcpu=False, remove_components = [
             LINKCOMSTR = 'LINK $TARGET'
         )
 
-    Repository(Rtt_Root)
-
     # we need to seperate the variant_dir for BSPs and the kernels. BSPs could
     # have their own components etc. If they point to the same folder, SCons
     # would find the wrong source code to compile.