Fix MinGW detection on x86

MinGW-w64 always uses "w64" as vendor key which the previously
used check for "64" anywhere in the target triplet matched. This
would lead to MinGW-w64 setups always being treated as x64.

Turns out we do not even need this additional check since the
architecture has been correctly determined earlier in this file.
No need to do it again.

cmake/OpenCVDetectCXXCompiler.cmake

@@ -147,11 +147,7 @@ if(MSVC)
 elseif(MINGW)
   set(OpenCV_RUNTIME mingw)
 
-  execute_process(COMMAND ${CMAKE_CXX_COMPILER} -dumpmachine
-                  OUTPUT_VARIABLE OPENCV_GCC_TARGET_MACHINE
-                  OUTPUT_STRIP_TRAILING_WHITESPACE)
-  if(CMAKE_OPENCV_GCC_TARGET_MACHINE MATCHES "64")
-    set(MINGW64 1)
+  if(MINGW64)
     set(OpenCV_ARCH x64)
   else()
     set(OpenCV_ARCH x86)