libparted: remove last_usable_if_grown

Commit 3398e82a: "libparted: Use common function to calculate PTE sectors"
removed usage of the last_usable_if_grown variable, resulting in an error
because it is now unused but still defined.
tests: fix t1700

bb181a7e: "tests: Use force for xfs in t1700 and a larger file" caused
the previous filesystem signatures to be left in the image file
causing mkfs.nilfs2 to complain that there is already an xfs filesystem
there, and hang the test suite waiting for an answer to proceed or not.
Remove the file between filesystems so it is recreated cleanly again.
tests: fix t2310-dos-extended-2-sector-min-offset.sh

This test was ignoring the requested sector size and always using
512 bytes per sector.  Fix it to use the requested sector size.
Fix filesystem detection on non 512 byte sectors

Enable probing for filesystems with non 512 byte sectors, and fix up each
filesystem to correctly handle that.  Remove unused field from the fs type
structure listing acceptable sector sizes.