systemc: Stop explicitly adding the systemc ext dir to CPPPATH.

That's now handled automatically when the native systemc API is
enabled.

Change-Id: If1f88929af335a40554a9fd8df6f68aa1a4c0ba4
Reviewed-on: https://gem5-review.googlesource.com/c/14398
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/tests/SConscript

@@ -61,7 +61,6 @@ if env['USE_SYSTEMC']:
                 'compile_only' : self.compile_only
             }
 
-    ext_dir = Dir('..').Dir('ext')
     test_dir = Dir('.')
     class SystemCTestBin(Executable):
         def __init__(self, test):
@@ -77,7 +76,6 @@ if env['USE_SYSTEMC']:
                 filter(lambda f: f not in to_remove, env['CCFLAGS'])
 
             env.Append(CPPPATH=test_dir.Dir('include'))
-            env.Append(CPPPATH=ext_dir)
 
             shared_lib_path = env['SHARED_LIB'][0].abspath
             sl_dir, sl_base = os.path.split(shared_lib_path)