doc/platform: Remove part about dllimport

Only deprecated things need this at all anymore, as we have functions
to access them or they are no longer needed.

The document should not encourage use of deprecated APIs.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

doc/platform.texi

@@ -179,16 +179,7 @@ If you plan to link with MSVC-built static libraries, you will need
 to make sure you have @code{Runtime Library} set to
 @code{Multi-threaded (/MT)} in your project's settings.
 
-Libav headers do not declare global data for Windows DLLs through the usual
-dllexport/dllimport interface. Such data will be exported properly while
-building, but to use them in your MSVC code you will have to edit the
-appropriate headers and mark the data as dllimport. For example, in
-libavutil/pixdesc.h you should have:
-@example
-extern __declspec(dllimport) const AVPixFmtDescriptor av_pix_fmt_descriptors[];
-@end example
-
-You will also need to define @code{inline} to something MSVC understands:
+You will need to define @code{inline} to something MSVC understands:
 @example
 #define inline __inline
 @end example