When calling DumpRepository and RestoreRepository on the same Gitea
instance, the users are preserved: all labels, issues etc. belong to
the external user who is, in this particular case, the local user.

Dead code verifying g.gitServiceType.Name() == "" (i.e. plain git) is
removed. The function is never called because the plain git downloader
does not migrate anything that is associated to a user, by definition.

Errors returned by GetUserIDByExternalUserID are no longer ignored.

The userMap is used when the external user is not kown, which is the
most common case. It was only used when the external user exists
which happens less often and, as a result, every occurence of an
unknown external user required a SQL query.

Signed-off-by: Loïc Dachary <loic@dachary.org>