build_style_name no longer strips Regular off of Condensed
fontmake now fails with MTI sources because `glyphsLib/builder/names/build_style_name()` no longer strips "Regular" from names like "NotoSansKhmer-Condensed". Now that gets written out to UFO as "NotoSansKhmer-CondensedRegular" and that key doesn't exist in the MTI plist. 

The old way for `build_style_name` was
```
if (italic or width or custom) and weight == 'Regular':
    weight = ''
```
but the new way doesn't remove "Regular":
```
    return ' '.join(
        s for s in (custom, width, weight, 'Italic' if is_italic else '') if s
    ) or 'Regular'
```
Is there a reason to keep Regular in the name? If so we need to update all the MTI plist files to expect the new name. If not we need to change this back to drop "Regular" like it used to. 
diff --git a/Lib/glyphsLib/builder/names.py b/Lib/glyphsLib/builder/names.py
index 3bb43dec..18063d37 100644
--- a/Lib/glyphsLib/builder/names.py
+++ b/Lib/glyphsLib/builder/names.py
@@ -26,7 +26,7 @@ def to_ufo_names(self, ufo, master, family_name):
 
     styleName = build_style_name(
         width if width != 'Regular' else '',
-        weight,
+        weight if weight != 'Regular' else '',
         custom,
         is_italic
     )
['Lib/glyphsLib/builder/names.py']