iwhois: remove iwhois plugin

The whois.geek.nz resolver is now dead.

Fixes #7297

plugins/iwhois/README.md

-# iwhois
-
-Provides a whois command with a more accurate and up-to-date list of whois servers
-using CNAMES, via [whois.geek.nz](https://github.com/iwantmyname/whois.geek.nz).
-
-To use it, add iwhois to the plugins array of your zshrc file:
-```
-plugins=(... iwhois)
-```
-
-### Usage
-
-The plugin defines the function `iwhois` that takes a domain name as an argument:
-
-```
-$ iwhois github.com
-   Domain Name: GITHUB.COM
-   Registry Domain ID: 1264983250_DOMAIN_COM-VRSN
-   Registrar WHOIS Server: whois.markmonitor.com
-   Registrar URL: http://www.markmonitor.com
-   Updated Date: 2017-06-26T16:02:39Z
-   Creation Date: 2007-10-09T18:20:50Z
-   ...
-```

plugins/iwhois/iwhois.plugin.zsh

-function iwhois() {
-    resolver="whois.geek.nz"
-    tld=`echo ${@: -1} | awk -F "." '{print $NF}'`
-    whois -h ${tld}.${resolver} "$@" ;
-}