kernFeatureWriter – streamline subtable report

[frankrolf]

When writing the kerning (with subtables) for a RTL font, the following output appears:

coverage table size: 1122
max subtable size: 8192
1 subtables created
coverage table size: 1122
max subtable size: 8192
3 subtables created
coverage table size: 1122
max subtable size: 8192
3 subtables created
coverage table size: 1122
max subtable size: 8192
3 subtables created

One for glyph/class, one for class/class, and two more for the RTL equivalent.
This is too much output, and not so interesting for the end user.