gdiplus plugin: remove MS Visual Studio 8 patches

gdiplus plugin: remove MS Visual Studio 8 patches

These patches appear to have been working around some non-portable code in the
headers shipped with Microsoft Visual Studio 8 (a.k.a. Microsoft Visual Studio
2005). This version of Visual Studio was EOLed on 2016-04-12. Neither the CMake
nor MS Build build systems (the only CI environments for which compilation of
this plugin is enabled) were using these patches. So apparently they have not
been useful for a long time.