doc: remove link to empty Go 1.5.3 milestone
doc: remove link to empty Go 1.5.3 milestone

Fixes #14687

Change-Id: I51fd8842d35c82db71cc5b28c4c0550871461eb6
Reviewed-on: https://go-review.googlesource.com/20421
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

miolini/go,Cofyc/go,cloudflare/go,nawawi/go,vsekhar/elastic-go,Cofyc/go,CAFxX/go,nawawi/go,Samurais/go,theif519/go,akutz/go,momchil-velikov/go,vanloswang/go,ImJasonH/go,vanloswang/go,4ad/go,momchil-velikov/go,linux-on-ibm-z/go,vsekhar/elastic-go,derekmarcotte/go,akutz/go,golang/go,vsekhar/elastic-go,josharian/go,momchil-velikov/go,momchil-velikov/go,miolini/go,golang/go,goodlang/good,linux-on-ibm-z/go,josharian/go,josharian/go,ImJasonH/go,Cofyc/go,vanloswang/go,codestation/go,akutz/go,ImJasonH/go,akutz/go,codestation/go,CAFxX/go,derekmarcotte/go,vsekhar/elastic-go,goodlang/good,derekmarcotte/go,nawawi/go,ImJasonH/go,goodlang/good,derekmarcotte/go,Cofyc/go,vsekhar/elastic-go,golang/go,nawawi/go,ImJasonH/go,Cofyc/go,miolini/go,ganboing/go-esx,Samurais/go,polyverse-security/go,josharian/go,vanloswang/go,derekmarcotte/go,4ad/go,vanloswang/go,polyverse-security/go,linux-on-ibm-z/go,Cofyc/go,josharian/go,akutz/go,nawawi/go,goodlang/good,miolini/go,nawawi/go,polyverse-security/go,polyverse-security/go,vsekhar/elastic-go,suncycheng/go,codestation/go,suncycheng/go,4ad/go,miolini/go,ganboing/go-esx,theif519/go,suncycheng/go,codestation/go,nawawi/go,suncycheng/go,vsekhar/elastic-go,ganboing/go-esx,theif519/go,linux-on-ibm-z/go,ImJasonH/go,4ad/go,golang/go,momchil-velikov/go,akutz/go,4ad/go,Samurais/go,CAFxX/go,vanloswang/go,miolini/go,polyverse-security/go,christopher-henderson/Go,CAFxX/go,polyverse-security/go,cloudflare/go,Samurais/go,miolini/go,miolini/go,suncycheng/go,polyverse-security/go,ImJasonH/go,ganboing/go-esx,goodlang/good,codestation/go,derekmarcotte/go,cloudflare/go,golang/go,4ad/go,theif519/go,goodlang/good,golang/go,goodlang/good,cloudflare/go,suncycheng/go,golang/go,Samurais/go,linux-on-ibm-z/go,ImJasonH/go,4ad/go,cloudflare/go,suncycheng/go,ganboing/go-esx,ganboing/go-esx,CAFxX/go,derekmarcotte/go,cloudflare/go,josharian/go,momchil-velikov/go,suncycheng/go,theif519/go,Cofyc/go,josharian/go,CAFxX/go,CAFxX/go,vsekhar/elastic-go,Samurais/go,CAFxX/go,polyverse-security/go,akutz/go,theif519/go,ganboing/go-esx,golang/go,codestation/go,linux-on-ibm-z/go,derekmarcotte/go,goodlang/good,nawawi/go,momchil-velikov/go,cloudflare/go,akutz/go,codestation/go,Samurais/go,linux-on-ibm-z/go,josharian/go,Samurais/go,codestation/go,Cofyc/go,ganboing/go-esx,theif519/go,linux-on-ibm-z/go,vanloswang/go,momchil-velikov/go,cloudflare/go,vanloswang/go,4ad/go,theif519/go