Remove very slow code that does nothing

[khaledhosny]

It is iterating over all masters in the font, then tries to access the master by its id and then checks if the master is None, but this can never happen since we are iterating over already existing masters.

This code is called in a very hot spot, and in a script I’m profiling, it takes more than 50% of the font build time.

Fixes https://github.com/googlefonts/glyphsLib/issues/646

[schriftgestalt]

I think the code would make some sense when it would iterate over the masters and checks for layers but as discussed not be issue, that is not really needed.