Kill WTF::DecimalNumber class

Source/wtf/wtf.gypi

 # For GN compatibility, this file can not have any conditions.
 {
     'variables': {
         'wtf_files': [
             'ASCIICType.h',
             'Alignment.h',
             'ArrayBuffer.cpp',
             'ArrayBuffer.h',
             'ArrayBufferBuilder.cpp',
             'ArrayBufferBuilder.h',
@@ skipping 19 matching lines @@
             'Compiler.h',
             'Complex.h',
             'CryptographicallyRandomNumber.cpp',
             'CryptographicallyRandomNumber.h',
             'CurrentTime.cpp',
             'CurrentTime.h',
             'DataLog.cpp',
             'DataLog.h',
             'DateMath.cpp',
             'DateMath.h',
-            'DecimalNumber.cpp',
-            'DecimalNumber.h',
             'DefaultAllocator.cpp',
             'DefaultAllocator.h',
             'Deque.h',
             'DoublyLinkedList.h',
             'DynamicAnnotations.cpp',
             'DynamicAnnotations.h',
             'FastAllocBase.h',
             'FastMalloc.cpp',
             'FastMalloc.h',
             'FilePrintStream.cpp',
@@ skipping 182 matching lines @@
             'text/TextCodecUTF8Test.cpp',
             'text/WTFStringTest.cpp',
         ],
         'wtf_unittest_helper_files': [
             'testing/WTFTestHelpers.cpp',
             'testing/WTFTestHelpers.h',
             'testing/WTFUnitTestHelpersExport.h',
         ],
     },
 }