Update vivado.py

Fix regression which caused Vivado to not be run at all.

litex/build/xilinx/vivado.py

@@ -243,11 +243,8 @@ class XilinxVivadoToolchain:
         if run:
             if synth_mode == "yosys":
                 common._run_yosys(platform.device, sources, platform.verilog_include_paths, build_name)
-            else:
-                raise OSError("Error!")
             _run_vivado(build_name, toolchain_path, source)
 
-
         os.chdir(cwd)
 
         return v_output.ns