Stop sending release events for mouse wheel 'buttons' in X mouse mode.

On Windows (X mouse reporting of the mouse wheel isn't currently done
by the Unix front end, though I'm shortly about to fix that too) a
mouse wheel event is translated into a virtual button, and we send
both a press and a release of that button to terminal.c, which encodes
both in X mouse reporting escape sequences and passes them on to the
server. This isn't consistent with what xterm does - scroll-wheel
events are encoded _like_ button presses, but differ semantically in
that they don't have matching releases. So we're updating to match
xterm.

[originally from svn r10138]

windows/window.c

@@ -3248,10 +3248,6 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
 				   TO_CHR_X(p.x),
 				   TO_CHR_Y(p.y), shift_pressed,
 				   control_pressed, is_alt_pressed());
-			term_mouse(term, b, translate_button(b),
-				   MA_RELEASE, TO_CHR_X(p.x),
-				   TO_CHR_Y(p.y), shift_pressed,
-				   control_pressed, is_alt_pressed());
 		    } /* else: not sure when this can fail */
 		} else {
 		    /* trigger a scroll */