graphics/graphite2: rely on default EXTRACT_SUFX after r411337

Mainly to fix 2016Q1 as USES=tar is nop with USE_GITHUB after r410613.

> silnrsi-graphite-1.3.7_GH0.tgz is not in graphics/graphite2/distinfo.

> Either graphics/graphite2/distinfo is out of date, or

> silnrsi-graphite-1.3.7_GH0.tgz is spelled incorrectly.

or

> Extracting for graphite2-1.3.7

> No SHA256 checksum recorded for silnrsi-graphite-1.3.7_GH0.tgz.

> No suitable checksum found for silnrsi-graphite-1.3.7_GH0.tgz.

Reported by: mav