refactor: replace all config.vitis_path with config.xilinx_path across codebase

Affected files:
- flash_programmer.py (2 call sites) — get_program_flash_path
- bitstream_programmer.py (3 get_xsct_path + 1 get_vivado_path call sites)
  Also: removed duplicate _get_vivado_path() (now uses shared one from vitis_checker)
  Also: removed unused shutil import
- reboot_manager.py (1 call site) — get_xsct_path

All now use getattr(config, 'xilinx_path', '') consistently.
The only remaining 'config.vitis_path' reference is in vitis_checker.py's
check_vitis() itself (safe legacy fallback for old configs).

src/reboot_manager.py

@@ -186,8 +186,7 @@ def reboot_via_jtag(
     from vitis_checker import get_xsct_path
 
     xsct = get_xsct_path(
-        vitis_path=config.vitis_path if hasattr(config, 'vitis_path') else "",
-        xilinx_root=config.xilinx_path if hasattr(config, 'xilinx_path') else "",
+        xilinx_root=getattr(config, "xilinx_path", ""),
     )
     if not xsct:
         return RebootResult(