MDEV-25341 post-fix. Don't use DiscardVirtualMemory on Windows.

This turns out to be a too expensive call to use for every freed page,
especially under global buffer pool mutex protection.

Also, the usefulness of proactive shrinking of the buffer pool on Windows
this is seems questionable - there is no OOM killer, and the OS will
shrink working sets itself.

storage/innobase/include/buf0buf.h

@@ -662,8 +662,6 @@ class buf_page_t
     MEM_NOACCESS(frame, srv_page_size);
 #ifdef MADV_FREE
     madvise(frame, srv_page_size, MADV_FREE);
-#elif defined(_WIN32)
-    DiscardVirtualMemory(frame, srv_page_size);
 #endif
   }