Erigon systemd service update (remove `--externalcl` flag from `erigon.service`)

[Griffin Ichiba Hotchkiss]

Following setup for Erigon as-is creates an error related to the --externalcl flag.

--externalcl without an argument will now cause erigon to halt (flag provided but not defined: -externalcl)

Removing the line entirely solves the issue as --externalcl is now default