ICU-21337 Remove U_I18N_API from enum.
diff --git a/icu4c/source/i18n/units_converter.h b/icu4c/source/i18n/units_converter.h
index 8676405..7650131 100644
--- a/icu4c/source/i18n/units_converter.h
+++ b/icu4c/source/i18n/units_converter.h
@@ -94,7 +94,7 @@
         : source(std::move(source)), target(std::move(target)) {}
 };
 
-enum U_I18N_API Convertibility {
+enum Convertibility {
     RECIPROCAL,
     CONVERTIBLE,
     UNCONVERTIBLE,