Revert "disk_util: use FAT32 on ESP"

This reverts commit 7f058d6.

Reverting because of bug 2284 [1] where grub will sometimes fail due to
memory corruption. This is _not_ the cause of the bug, and the bug can
even be reproduced with this reversion, but it seems to occur less when
not using fat32.

[1] coreos/bugs#2284

build_library/disk_util

@@ -423,9 +423,6 @@ def FormatFat(part, device):
   cmd = ['mkfs.vfat']
   if 'fs_label' in part:
     cmd += ['-n', part['fs_label']]
-  if part['type'] == 'efi':
-    # ESP is FAT32 irrespective of size
-    cmd += ['-F', '32']
   Sudo(cmd + [device, vfat_blocks], stdout_null=True)