remove obsolete fdisk hack
8321653c (from 2007) introduced a hack to work around bug #445304 in gnu-fdisk for users who may have replaced fdisk with the classic gnu version. the hack allowed users to select an alternate fdisk binary to use to work around the buggy binary.
bug #445304 is marked as found in v1.0-1 and fixed in v1.2-1, though may have been fixe din v1.1. it was marked fixed in 2009.
checking the package archive, gnu-fdisk does not actually exist anymore in debian, with one exception - it is available for arm64 on sid via debports, and that version is 1.3 so thus includes the necessary fix anyway.
it is thus pointless now that we still carry this hack.
Gbp-Dch: Short