fixes undesirable lines. Fixes #62483

What does this MR do?

Removes undesired lines in screen pipeline/new separating variables field

As commented by @dimitrieh in !27473 (closed) (comment 173809577):

Follows a list of the three views that use /ci/_variable_row.html.haml, which added the undesired lines:

Screenshot of pipeline_schedules/new

Screenshot of pipelines/new

Screenshot of settings/ci_cd