eutils.eclass: prune_libtool_files, punt pointless Prefix logic

eutils.eclass: prune_libtool_files, punt pointless Prefix logic

Remove the unnecessary Prefix logic from prune_libtool_files(). There is
no functional difference between starting a find in ${D} and ${ED}
(since ${D} is not supposed to contain other directories on a Prefix
system), and using the latter implies unnecessary hackery for older
EAPIs.