Fix fixtures: remove unnecessary build flags, use C++17 namespace style

tests/integration/fixtures/external_components/legacy_climate_component/climate/legacy_climate.h

@@ -3,8 +3,7 @@
 #include "esphome/components/climate/climate.h"
 #include "esphome/core/component.h"
 
-namespace esphome {
-namespace legacy_climate_test {
+namespace esphome::legacy_climate_test {
 
 /// Test climate that uses the DEPRECATED ClimateTraits setters for custom modes.
 /// This validates backward compatibility for external components that haven't migrated.
@@ -55,5 +54,4 @@ class LegacyClimate : public climate::Climate, public Component {
   }
 };
 
-}  // namespace legacy_climate_test
-}  // namespace esphome
+}  // namespace esphome::legacy_climate_test

tests/integration/fixtures/legacy_climate_compat.yaml

@@ -1,8 +1,5 @@
 esphome:
   name: legacy-climate-compat
-  platformio_options:
-    build_flags:
-      - "-DUSE_HOST"
 
 host:
 api: