Remove unnecessary headers from strutil.cpp causing build trouble (#3976)

While testing out the 2.5 beta for use in Blender, I hit an odd build
error[1] apparently related to include order. Apparently `windows.h`
should be included before `shellapi.h`[2]. Swapping the order does fix
things, however, it turns out neither is used in this particular file so
just remove them both. Unsure why we didn't see it until now.

[1]
```
20>[9/146] Building CXX object src\libutil\CMakeFiles\OpenImageIO_Util.dir\strutil.cpp.obj
20>FAILED: src/libutil/CMakeFiles/OpenImageIO_Util.dir/strutil.cpp.obj

20>cl : Command line warning D9002: ignoring unknown option '/arch:sse2'
20>C:\Program Files (x86)\Windows Kits\10\Include\10.0.22621.0\um\shellapi.h(68): error C2146: syntax error: missing ';' before identifier 'DECLSPEC_IMPORT'
20>C:\Program Files (x86)\Windows Kits\10\Include\10.0.22621.0\um\shellapi.h(79): error C2065: 'HDROP': undeclared identifier
20>C:\Program Files (x86)\Windows Kits\10\Include\10.0.22621.0\um\shellapi.h(82): error C2086: 'int EXTERN_C': redefinition
20>C:\Program Files (x86)\Windows Kits\10\Include\10.0.22621.0\um\shellapi.h(68): note: see declaration of 'EXTERN_C'
```
[2] 

https://stackoverflow.com/questions/2953704/vs2010-lots-of-errors-when-including-standard-libraries

https://social.msdn.microsoft.com/forums/vstudio/en-us/ed93ae48-969a-4280-8fd5-ef57a332022b/problems-with-shellexecute-and-shellapih?forum=vcgeneral

## Checklist:

<!-- Put an 'x' in the boxes as you complete the checklist items -->

- [X] I have read the [contribution
guidelines](https://github.com/OpenImageIO/oiio/blob/master/CONTRIBUTING.md).
- [ ] I have updated the documentation, if applicable.
- [ ] I have ensured that the change is tested somewhere in the
testsuite
  (adding new test cases if necessary).
- [ ] If I added or modified a C++ API call, I have also amended the
corresponding Python bindings (and if altering ImageBufAlgo functions,
also
  exposed the new functionality as oiiotool options).
- [X] My code follows the prevailing code style of this project. If I
haven't
already run clang-format before submitting, I definitely will look at
the CI
test that runs clang-format and fix anything that it highlights as being
  nonconforming.

Signed-off-by: Jesse Yurkovich <jesse.y@gmail.com>

Author: jessey-git

src/libutil/strutil.cpp

@@ -31,11 +31,6 @@
 #include <OpenImageIO/thread.h>
 #include <OpenImageIO/ustring.h>
 
-#ifdef _WIN32
-#    include <shellapi.h>
-#    include <windows.h>
-#endif
-
 
 // We use the public domain stb implementation of vsnprintf because
 // not all platforms support a locale-independent version of vsnprintf.