moc: Only generate IndexOfMethod for signals.

moc: Only generate IndexOfMethod for signals.

moc is currently generating code to convert from a pointer to member
function of a slot or signal to its index.

The idea was that it could be usefull for slots to have the new syntax
do the same as the old one (connecting signal index to slot index). But
in practice, the new syntax do not use the IndexOfMethod for slots.

Also, it does not work for all the slots (no Q_PRIVATE_SLOT,
no static slots)

So since it is not used, and that it would take room in the binaries to
generate all the code to get the index of slots, we remove it.

If ever we need it, we can still add it later.

Change-Id: Ia417e3e524d7915ca86433ea86c66ac2b299c81a
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>