Workaround to allow openzfs-zfs-dkms install on Ubuntu

As shown in #15404#issuecomment-1765002181, Ubuntu kernel has
'Provides: zfs-dkms', which will cause uninstall of the kernel, when
attempting to install openzfs-zfs-dkms.
As a workaround remove the 'Conflicts: zfs-dkms' definition from
the debian control file.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mart Frauenlob <AllKind@fastest.cc>
Closes #15503