Drop dead -I build-flag workarounds now that vendor/CMakeLists.txt is correct

These global cg.add_build_flag("-I...") calls predate understanding
that vendor is a component-manager dependency of the main app
component (via add_idf_component + src/idf_component.yml), and that
its own CMakeLists.txt is the right place to declare the include dirs
consumers need (public) versus only vendor's own sources need
(private) - see 12b1119 and 4144bfd.

They're now doing nothing on either toolchain: ESPHome's native
esp-idf toolchain drops -I flags from its project-wide COMPILE_OPTIONS
propagation entirely (only -D/-W survive), and PlatformIO's copy is
redundant with what the CMakeLists already declares. Verified all 9
test fixtures, PlatformIO and native toolchain alike, still build
clean without them.

components/gxepd2/display.py

@@ -32,7 +32,6 @@ DEPENDENCIES = ["spi"]
 CONF_CS2_PIN = "cs2_pin"
 
 VENDOR_ROOT = Path(__file__).parent / "vendor"
-ARDUINO_COMPAT_DIR = Path(__file__).parent / "arduino_compat"
 CATALOG_PATH = Path(__file__).parent / "catalog.json"
 CATALOG = json.loads(CATALOG_PATH.read_text())
 
@@ -122,8 +121,6 @@ async def to_code(config: ConfigType) -> None:
 
     add_idf_component(name=VENDOR_ROOT.name, path=str(VENDOR_ROOT))
     _write_sources_manifest(entry["source"])
-    cg.add_build_flag(f"-I{ARDUINO_COMPAT_DIR}")
-    cg.add_build_flag(f"-I{CORE.relative_src_path()}")
 
     cs_id, dc_id = _allocate_pin_id(), _allocate_pin_id()
     reset_id = _allocate_pin_id() if CONF_RESET_PIN in config else -1