dnsmasq/shared: drop "--dhcp-lease-max=50" option

It's not clear why this option is set or why it's necessary. It was
set ever since.

Note that we automatically determine the DHCP range
nm_dnsmasq_utils_get_range(), and with /24 subnets (or larger)
the range spans up to 244 addresses.

With such a configuration it doesn't seem to make sense to limit the
number of leases.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/ ## 941

src/core/dnsmasq/nm-dnsmasq-manager.c

@@ -188,8 +188,6 @@ create_dm_cmd_line(const char           *iface,
         nm_strv_ptrarray_add_string_dup(cmd, "--dhcp-option-force=43,ANDROID_METERED");
     }
 
-    nm_strv_ptrarray_add_string_dup(cmd, "--dhcp-lease-max=50");
-
     nm_strv_ptrarray_add_string_printf(cmd,
                                        "--dhcp-leasefile=%s/dnsmasq-%s.leases",
                                        NMSTATEDIR,