docs: remove old client-matrix link
docs: remove old client-matrix link

This document was removed in 23406dc2ee261dcdcfd35b2dbad29a55e09bc9fc
but still linked to here.

purpleidea/etcd,fanminshi/etcd,sofuture/etcd,swingbach/etcd,insequent/etcd,handsomesun/etcd,etcd-io/etcd,dnaeon/etcd,joshix/etcd,ravigadde/etcd,OyTao/etcd_learning,YuleiXiao/etcd,chipironcin/etcd,linuxcer/etcd,siddontang/etcd,whuwxl/etcd,mkumatag/etcd,li-ang/etcd,CrazyJvm/etcd,gyuho/etcd,bluebreezecf/etcd,bdarnell/etcd,StorReduce/etcd,ajityagaty/etcd,dnaeon/etcd,lorneli/etcd,glycerine/etcd,visheshnp/etcd,luxas/etcd,OyTao/etcd_learning,dannysauer/etcd,tguangch/etcd,armstrongli/etcd,purpleidea/etcd,heyitsanthony/etcd,tguangch/etcd,armstrongli/etcd,xiang90/etcd,yudai/etcd,akauppi/etcd,mqliang/etcd,cockroachdb/etcd,hongchaodeng/etcd,bluebreezecf/etcd,westhood/etcd,jlamillan/etcd,sinsharat/etcd,ajityagaty/etcd,rchicoli/etcd,ajityagaty/etcd,xiang90/etcd,sebrandon1/etcd,linuxcer/etcd,wantonsolutions/dinv-etcd,qqsun8819/etcd,westhood/etcd,glevand/coreos--etcd,plasticbox/etcd,chipironcin/etcd,vimalk78/etcd,mkumatag/etcd,CrazyJvm/etcd,achanda/etcd,mickep76/etcd,glycerine/etcd,nekto0n/etcd,mqliang/etcd,rnaveiras/etcd,vimalk78/etcd,rnaveiras/etcd,wantonsolutions/dinv-etcd,ktateish/etcd,fanminshi/etcd,gyuho/etcd,wantonsolutions/dinv-etcd,etcd-io/etcd,whuwxl/etcd,ktateish/etcd,bdarnell/etcd,jonboulle/etcd,tamird/etcd,heyitsanthony/etcd,YuleiXiao/etcd,rchicoli/etcd,achanda/etcd,lumjjb/etcd,ravigadde/etcd,siddontang/etcd,luxas/etcd,a-robinson/etcd,visheshnp/etcd,dannysauer/etcd,mqliang/etcd,jlamillan/etcd,cockroachdb/etcd,glevand/coreos--etcd,etcd-io/etcd,swingbach/etcd,mkumatag/etcd,insequent/etcd,lumjjb/etcd,a-robinson/etcd,yudai/etcd,joshix/etcd,endocode/etcd,endocode/etcd,sofuture/etcd,nekto0n/etcd,zaphod-concur/etcd,rnaveiras/etcd,li-ang/etcd,tamird/etcd,coreos/etcd,StorReduce/etcd,plasticbox/etcd,jonboulle/etcd,qqsun8819/etcd,sinsharat/etcd,zaphod-concur/etcd,lorneli/etcd,akauppi/etcd,handsomesun/etcd,hongchaodeng/etcd,sebrandon1/etcd,mickep76/etcd,coreos/etcd