Revert "wic: Set a miniumum FAT16 volume size."

Revert "wic: Set a miniumum FAT16 volume size."

This reverts commit f7dfb4d43247d3c13a4e0a3853007d63b9512b83.

FAT16 volumes do not have a minimum size of 8250 blocks
(== slightly over 4MB).  Exhibit A: floppy disks.

The original commit message suggests this was a workaround for a bug in
parted - in which case we should fix it there, or use the wic
`--fixed-size` option to pass down an enforced minimum from a more
context-aware point in the callstack.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>