[core] Remove cv.only_with_esp_idf and CORE.using_esp_idf (#16681)

script/ci-custom.py

@@ -693,19 +693,6 @@ def lint_esphome_h(fname, line, col, content):
     )
 
 
-@lint_content_find_check(
-    "CORE.using_esp_idf",
-    include=py_include,
-    exclude=["esphome/core/__init__.py", "script/ci-custom.py"],
-)
-def lint_using_esp_idf_deprecated(fname, line, col, content):
-    return (
-        f"{highlight('CORE.using_esp_idf')} is deprecated and will change behavior in 2026.6. "
-        "ESP32 Arduino builds on top of ESP-IDF, so ESP-IDF features are available in both frameworks. "
-        f"Please use {highlight('CORE.is_esp32')} and/or {highlight('CORE.using_arduino')} instead."
-    )
-
-
 @lint_content_check(include=["*.h"], exclude=["esphome/core/entity_types.h"])
 def lint_pragma_once(fname, content):
     if "#pragma once" not in content: