Remove superfluous Ahem.tff

This was added in https://mondrian.corp.google.com/changelist/13754-p9
and comes from the WebKit LayoutTests. It's no longer used anywhere so
is being removed. It will be added in external/webkit/LayoutTests when
it is required for the triaged LayoutTests.

Bug: 3009858
Change-Id: I4b00b3f435fee9f8018052f4f8a5f3f67795437f