DNS: move nsupdate.info/SPF toolbox to awesome-sysadmin/DNS - Control Panels & Domain Management (#3940)

- ref. https://github.com/awesome-foss/awesome-sysadmin/pull/504
- ref. https://github.com/awesome-foss/awesome-sysadmin/pull/473
- ref. https://github.com/awesome-selfhosted/awesome-selfhosted/pull/3938

README.md

@@ -756,9 +756,7 @@ _See also: [awesome-sysadmin/DNS - Servers](https://github.com/awesome-foss/awes
 
 - [blocky](https://github.com/0xERR0R/blocky) - Fast and lightweight DNS proxy (like Pi-hole) as ad-blocker for local network with many features. `Apache-2.0` `Go`
 - [Maza ad blocking](https://maza-ad-blocking.andros.dev/) - Local ad blocker. Like Pi-hole but local and using your operating system. ([Source Code](https://github.com/tanrax/maza-ad-blocking)) `Apache-2.0` `Bash`
-- [nsupdate.info](https://www.nsupdate.info/) - Dynamic DNS service. ([Demo](https://www.nsupdate.info/account/register/), [Source Code](https://github.com/nsupdate-info/nsupdate.info)) `BSD-3-Clause` `Python`
 - [SmartDNS](https://github.com/pymumu/smartdns) - A local DNS server to obtain the fastest website IP for the best Internet experience, supports DoT, DoH. `GPL-3.0` `C`
-- [SPF Toolbox](https://spftoolbox.com) - Application to look up DNS records such as SPF, MX, Whois, and more. ([Source Code](https://github.com/charlesabarnes/SPFtoolbox)) `MIT` `PHP`
 - [Technitium DNS Server](https://technitium.com/dns/) - An open source authoritative as well as recursive DNS server that can be used for self hosting a DNS server for privacy & security. ([Source Code](https://github.com/TechnitiumSoftware/DnsServer)) `GPL-3.0` `Docker/C#`