Remove dedicated TextCodecUTF16

Source/wtf/wtf.gypi

 # For GN compatibility, this file can not have any conditions.
 {
     'variables': {
         'wtf_files': [
             'ASCIICType.h',
             'AddressSpaceRandomization.cpp',
             'AddressSpaceRandomization.h',
             'Alignment.h',
             'ArrayBuffer.cpp',
             'ArrayBuffer.h',
@@ skipping 168 matching lines @@
             'text/StringOperators.h',
             'text/StringStatics.cpp',
             'text/StringUTF8Adaptor.h',
             'text/StringView.h',
             'text/TextCodec.cpp',
             'text/TextCodecASCIIFastPath.h',
             'text/TextCodecICU.cpp',
             'text/TextCodecLatin1.cpp',
             'text/TextCodecReplacement.cpp',
             'text/TextCodecReplacement.h',
-            'text/TextCodecUTF16.cpp',
             'text/TextCodecUTF8.cpp',
             'text/TextCodecUTF8.h',
             'text/TextCodecUserDefined.cpp',
             'text/TextEncoding.cpp',
             'text/TextEncodingRegistry.cpp',
             'text/TextPosition.cpp',
             'text/TextPosition.h',
             'text/WTFString.cpp',
             'text/WTFString.h',
             'unicode/CharacterNames.h',
@@ skipping 33 matching lines @@
             'text/TextCodecUTF8Test.cpp',
             'text/WTFStringTest.cpp',
         ],
         'wtf_unittest_helper_files': [
             'testing/WTFTestHelpers.cpp',
             'testing/WTFTestHelpers.h',
             'testing/WTFUnitTestHelpersExport.h',
         ],
     },
 }