bsp: cvitek: clean up useless variable assignments in SConstruct

CV18xx RISC-V C906L.

Configuration value for CPU and ARCH are already defined in rtconfig.py.
Remove these duplication from SConstruct.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

bsp/cvitek/cv18xx_risc-v/SConstruct

@@ -21,8 +21,6 @@ env['ASCOM'] = env['ASPPCOM']
 
 Export('RTT_ROOT')
 Export('rtconfig')
-rtconfig.CPU='virt64'
-rtconfig.ARCH='risc-v'
 
 SDK_ROOT = os.path.abspath('./')