libquota: remove quota_is_on() which was the last user of quotactl()

libquota: remove quota_is_on() which was the last user of quotactl()

The quotactl() system call was being used without the use of a
function prototype.  On closer examination, it turns out the one user
of that system call was the quota_is_on() function, which is not used
by e2fsprogs at all.  Since libquota is an e2fsprogs-internal library,
and not one that we plan to export any time soon, the simplest thing
to do is to simply remove quota_is_on(), which in turn allows us to
remove all of the infrastructure around using the Linux-specific
quotactl() system call.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>