Remove mutex for retrieving default typeface.

This mutex was added due to FontConfigTypeface::LegacyCreateTypeface
being non-thread safe. This method no longer exists and the logic was
moved to SkFontMgr_FCI::onLegacyCreateTypeface which has access to an
appropriate mutex ("Clean up SkFontConfigInterface implementation.",
0265707c191a31dfde08dd1cd7011c1fe5b8e643).

Review-Url: https://codereview.chromium.org/1994703003
1 file changed