dhcp: drop dhcpcanon support

Drop support for the "dhcpcanon" DHCP client. It's unmantained, as the
last code change was in 2018:

  https://github.com/juga0/dhcpcanon/commits

There is no need to first deprecate it because it was still marked as
"experimental" in NM. Also, it's not packaged by any recent distro, so
we can assume that nobody will miss it.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/2112

contrib/fedora/rpm/NetworkManager.spec

@@ -602,7 +602,6 @@ Preferably use nmcli instead.
 %else
 	-Ddhclient=no \
 %endif
-	-Ddhcpcanon=no \
 	-Ddhcpcd=no \
 	-Dcrypto=gnutls \
 %if %{with debug}

contrib/fedora/rpm/configure-for-system.sh

@@ -369,7 +369,6 @@ meson setup\
     -Dnft="${D_SBINDIR}/nft" \
     -Diptables="${D_SBINDIR}/iptables" \
     -Ddhclient="${D_SBINDIR}/dhclient" \
-    -Ddhcpcanon=no \
     -Ddhcpcd=no \
     -Dconfig_dhcp_default="$P_DHCP_DEFAULT" \
     "-Dcrypto=$P_CRYPTO" \

contrib/scripts/nm-ci-run.sh

@@ -173,7 +173,6 @@ meson setup build \
     -D teamdctl=$_WITH_LIBTEAM \
     \
     -D dhclient=/bin/nowhere/dhclient \
-    -D dhcpcanon=/bin/nowhere/dhcpcanon \
     -D dhcpcd=/bin/nowhere/dhcpd \
     \
     -D netconfig=/bin/nowhere/netconfig \