debian/control: remove unneeded dependency on network-manager.

[menakite]

It was added in 8021bb1 as an alternative to dhcpcd5, but when dhcpcd5 was removed in 1a37b2b network-manager was left there.

Fixes removing network-manager without uninstalling raspberrypi-net-mods too:

menakite@raspy:~ $ sudo apt remove --purge network-manager
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following packages were automatically installed and are no longer required:
  dns-root-data dnsmasq-base libbluetooth3 libcurl3-gnutls libmm-glib0 libndp0 libnetfilter-conntrack3 libnfnetlink0 libnm0 libteamdctl0
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  network-manager* raspberrypi-net-mods*
0 upgraded, 0 newly installed, 2 to remove and 0 not upgraded.
After this operation, 16.1 MB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
menakite@raspy:~ $

[XECDesign]

Why would you note remove raspberrypi-net-mods in this situation?

Currently, it's only used to let the user know that their wifi won't work unless they set the regulatory domain through raspi-config. The assumption on our end is that if the user is running Raspberry Pi OS, then they're probably using NetworkManager. If not, then we can't test that our changes work as expected and there's no reason for this package to be installed, right?