Remove st_blocks hack from wxFile::Length() under Linux

This reverts commit 41f6f17d01 ("return 0
(meaning the file is not seekable, as the docs now explain) instead of
4KB for the files in sysfs under Linux") as it seems to be wrong to
return a value different from what "ls -l" or "stat" return here and the
original problem was solved in a better way in the previous commit.

See #9965.

Closes #17818.

interface/wx/file.h

@@ -95,11 +95,14 @@ public:
     /**
         Returns the length of the file.
 
-        This method may return ::wxInvalidOffset if the length couldn't be
-        determined or 0 even for non-empty files if the file is not seekable.
+        Returns ::wxInvalidOffset if the length couldn't be determined.
 
-        In general, the only way to determine if the file for which this function
-        returns 0 is really empty or not is to try reading from it.
+        Please also note that there is @e no guarantee that reading that many
+        bytes from the file will always succeed. While this is true for regular
+        files (unless the file size has been changed by another process in
+        between Length() and Read() calls), some special files, such as most
+        files under @c /sys or @c /proc directories under Linux, don't actually
+        contain as much data as their size indicates.
     */
     wxFileOffset Length() const;