Revert "Make GlyphOrder object iterable"

This reverts commit e4a670cc7da93d3a12ba23d8cfefdeb0ec7be01f.

As Read Roberts wrote to me:

"you changed the definition of the GlyphOrder class to take a ttFont as
the argument for the __init__ function, rather than just the
tag, as before, I think so that the glyph order is defined when the
table is instantiated, rather than only when to/fromXML() is called

The problem with this is that the ttx.py compile function passes in a tag,
so compiling a font from an ttx file fails here, and in
xmlImport.startElementHandler(). I discovered this because a number of my
scripts use the same logic. What is the reason for this change? I have no
problem with changing the several FDK scripts that build a new TTF font
from scratch, to pass in the ttFont rather than a tag, but wanted to be
sure that this was necessary. The main issues are that when reading in an
entire TTX file, the table has to be instantiated before the data can be
provided, and the GylphOrder initialization is then different than  for
all the other tables"""

As such revert.  This means that GlyphOrder is again non-iterable.  Will
have to fix in some other way later.