Remove "can primary connect to secondary" check from gitlab:geo:check

Consider this output from gitlab-rake gitlab:geo:check:

GitLab Geo HTTP(S) connectivity ... 
* Can connect to secondary node: 'https://gprd.gitlab.com/' ... no

Historically, this check was there to ensure the primary could pull status information from the secondary.

Starting in 11.1 or so, the secondary began pushing status information to the primary, meaning this check no longer reflects a requirement of Geo. All HTTP traffic is with the primary as a server, and the secondary as a client, so we can remove this check.

It's still sensible to check that the secondary can connect to the primary over HTTP, however. On the secondary, that check currently looks like:

GitLab Geo HTTP(S) connectivity ... 
* Can connect to the primary node ... yes

/cc @ash.mckenzie @akaemmerle