Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1749841

Save button did not respond properly on changes in NTP Servers section while editing an existing Zone. Let me explain the fix. Here, the same as in other screens, values of @edit[:new] and @edit[:current] are crucial for detecting changes in the fields and response of Add/Save buttons. In our case, we need to take care of @edit[:new][:ntp] and @edit[:current][:ntp].
Beginning of editing a Zone:

[18] pry(#<OpsController>)> @edit[:new][:ntp]
=> {:server=>["0.pool.ntp.org", "1.pool.ntp.org", "2.pool.ntp.org"]}
[19] pry(#<OpsController>)> @edit[:current][:ntp]
=> {:server=>["0.pool.ntp.org", "1.pool.ntp.org", "2.pool.ntp.org"]}

After we make some change and put it back, the result will look like this:

[6] pry(#<OpsController>)> @edit[:new][:ntp]
=> {:server=>["0.pool.ntp.org", "1.pool.ntp.org", "2.pool.ntp.org"], "ntp_server_2"=>"1.pool.ntp.org"}
[7] pry(#<OpsController>)> @edit[:current][:ntp]
=> {:server=>["0.pool.ntp.org", "1.pool.ntp.org", "2.pool.ntp.org"]}

We are adding something extra to @edit[:new][:ntp]. We always just add something, not remove. And this is core of the problem why Save button remained always enabled if we touched anything in NTP Servers section, no matter we really made a change or not.

So we need to remove unnecessary key "ntp_server_2" from @edit[:new][:ntp] to achieve proper response of Save button. And as we can see that the field/server numbers in the loop are from 1 to 3, we have to look at field_num - 1th item in the array of @edit[:new][:ntp][:server] to remove the right key from @edit[:new][:ntp] as the second position (field_num == 2) in array is in position 1, obviously.

After: (Save button becomes disabled after we put back the original values in fields in NTP Servers)