Fix incorrect `page_title`, `nav`, and `header_title` usage in views

During https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19766,
several usages of these helper methods were mistakenly changed to
instead assign to a (completely unused) local variable.

When `page_title` was misused, for example, this resulted in the page no
longer having a custom title set.

app/views/layouts/explore.html.haml

@@ -1,5 +1,5 @@
-- page_title    = _("Explore")
+- page_title _("Explore")
 - unless current_user
-  - header_title  = _("Explore GitLab"), explore_root_path
+  - header_title _("Explore GitLab"), explore_root_path
 
 = render template: "layouts/application"

app/views/layouts/group_settings.html.haml

@@ -1,4 +1,4 @@
-- page_title    = _("Settings")
-- nav           "group"
+- page_title _("Settings")
+- nav "group"
 
 = render template: "layouts/group"

app/views/projects/pipelines/new.html.haml

@@ -1,5 +1,5 @@
 - breadcrumb_title "Pipelines"
-- page_title = s_("Pipeline|Run Pipeline")
+- page_title s_("Pipeline|Run Pipeline")
 - settings_link = link_to _('CI/CD settings'), project_settings_ci_cd_path(@project)
 
 %h3.page-title