graphics/potrace: Remove the recently added option LIBRARY because it directly conflicts with graphics/libpotrace

graphics/libpotrace and graphics/potrace build different parts
of the same project and should probably be merged. For now, delete the
offending option because it isn't needed.

Reported by: Jan Henrik Sylvester <me@janh.de>
Approved by: tcberner (mentor, implicit)
Approved by: portmgr (port compliance, infrastructure)