Simplify chip_variant initialization in PathCache

Removed redundant assignment of chip_variant.

Author: Jason2866

builder/frameworks/arduino.py

@@ -288,8 +288,6 @@ class PathCache:
     def __init__(self, platform, mcu, chip_variant):
         self.platform = platform
         self.mcu = mcu
-        chip_variant = env.BoardConfig().get("build.chip_variant", "").lower()
-        chip_variant = chip_variant if chip_variant else mcu
         self.chip_variant = chip_variant
         self._framework_dir = None
         self._framework_lib_dir = None