Remove obsolete wxUSE_IOSTREAMH option

This is always 0 for any still supported compiler, so remove the option
and configure checks for it.

Still define it as 0 for compatibility, just in case it's used outside
of the library.

src/common/variant.cpp

@@ -29,11 +29,7 @@
 #endif
 
 #if wxUSE_STD_IOSTREAM
-    #if wxUSE_IOSTREAMH
-        #include <fstream.h>
-    #else
-        #include <fstream>
-    #endif
+    #include <fstream>
 #endif
 
 #if wxUSE_STREAMS