AK: Remove now-unused UTF-16 length from UTF-8 string helper

Author: https://github.com/trflynn89
Commit: 6d1f90c739
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5720
Reviewed-by: https://github.com/gmta ✅

Tests/AK/TestUtf16View.cpp

@@ -86,14 +86,6 @@ TEST_CASE(decode_utf16)
     EXPECT_EQ(i, expected.size());
 }
 
-TEST_CASE(utf16_code_unit_length_from_utf8)
-{
-    EXPECT_EQ(AK::utf16_code_unit_length_from_utf8(""sv), 0uz);
-    EXPECT_EQ(AK::utf16_code_unit_length_from_utf8("abc"sv), 3uz);
-    EXPECT_EQ(AK::utf16_code_unit_length_from_utf8("😀"sv), 2uz);
-    EXPECT_EQ(AK::utf16_code_unit_length_from_utf8("Привет, мир! 😀 γειά σου κόσμος こんにちは世界"sv), 39uz);
-}
-
 TEST_CASE(null_view)
 {
     Utf16View view;