EOS_HTTP_THREADPOOL settings

We have

./fst/templates/configmap-env.yaml:  EOS_HTTP_THREADPOOL: "epoll"
./fst/templates/configmap-env.yaml:  EOS_HTTP_THREADPOOL_SIZE: !!integer 16
./mgm/templates/configmap-env.yaml:  EOS_HTTP_THREADPOOL: "epoll"
./mgm/templates/configmap-env.yaml:  EOS_HTTP_THREADPOOL_SIZE: !!integer 16

These variables are still functional, but based on dss/eos@79e96672 maybe we should not change them anymore? The default threadpool mode is 'threads', not 'epoll'.

https://gitlab.cern.ch/dss/eos/-/blob/master/common/http/HttpServer.cc?ref_type=heads#L70