Add only the origin remote to the local project

create_local_project no longer adds a github remote, with or without GitHub:
a push to origin reaches GitHub through the push mirror. A github remote that
already exists, such as one made by an earlier version, is left as it is, and
a different origin is still refused.

- Remove github_remote_url, which nothing else used
- README describes the one remote and how to remove an old github remote
- Tests: origin is the only remote with and without GitHub, no GitHub address
  in .git/config, and an existing github remote is kept

Task: MIL-008#1
Task: MIL-008#2
Task: MIL-008#3

src/lib/hosts.sh

@@ -4,7 +4,7 @@
 #
 # Part of create-project.sh: sourced by it, never run on its own.
 #
-# Provides: is_reused, repo_owner, repo_url, gitea_host, origin_protocol, origin_url, github_remote_url, framework_url
+# Provides: is_reused, repo_owner, repo_url, gitea_host, origin_protocol, origin_url, framework_url
 
 # is_reused HOST: succeed if the existing repository on HOST will be reused.
 is_reused() {
@@ -54,10 +54,6 @@ origin_url() {
   fi
 }
 
-github_remote_url() {
-  printf '%s/%s/%s.git' "${CONFIG[GITHUB_WEB_URL]}" "${PROJECT[github_owner]}" "${PROJECT[name]}"
-}
-
 # framework_url: where the framework submodule comes from (always SSH).
 framework_url() {
   printf 'ssh://git@%s:%s/%s.git' "$(gitea_host)" "${CONFIG[GITEA_SSH_PORT]}" \