remove unused methods

[kstenschke]

ccpcheck reports:

[src/docxbox/helper/helper_image.cc:9]: (style) The function 'GetDimension' is never used.
[src/docxbox/helper/helper_string.cc:102]: (style) The function 'GetSubStrAfter' is never used.
[src/docxbox/helper/helper_string.cc:28]: (style) The function 'GetSubStrCount' is never used.
[src/vendor/tinyxml2/tinyxml2.cpp:1979]: (style) The function 'InsertNewChildElement' is never used.
[src/vendor/tinyxml2/tinyxml2.cpp:1985]: (style) The function 'InsertNewComment' is never used.
[src/vendor/tinyxml2/tinyxml2.cpp:1997]: (style) The function 'InsertNewDeclaration' is never used.
[src/vendor/tinyxml2/tinyxml2.cpp:1991]: (style) The function 'InsertNewText' is never used.
[src/vendor/tinyxml2/tinyxml2.cpp:2003]: (style) The function 'InsertNewUnknown' is never used.
[src/docxbox/helper/helper_file.cc:6]: (style) The function 'IsFile' is never used.
[src/docxbox/helper/helper_string.cc:123]: (style) The function 'IsNumeric' is never used.
[src/docxbox/helper/helper_string.cc:60]: (style) The function 'ReplaceAll' is never used.
[src/docxbox/helper/helper_string.cc:38]: (style) The function 'ReplaceFirstOccurrence' is never used.
[src/docxbox/helper/helper_string.cc:155]: (style) The function 'ToLower' is never used.
[src/docxbox/helper/helper_string.cc:163]: (style) The function 'ToUpper' is never used.
[src/docxbox/helper/helper_string.cc:168]: (style) The function 'UcFirst' is never used.