docker: remove scip-ctags from Dockerfile (#802)
This is an optional dependency that I don't believe OSS users of docker
use. We used to use these dockerfiles in Sourcegraph, but have since
moved to building it in our own CI with scip-ctags.
Test Plan: built locally
docker build -t zoekt .
docker build -t zoekt-indexserver . -f Dockerfile.indexserver
docker build -t zoekt-webserver . -f Dockerfile.webserver