Revert "Defend against 'Host' header injection"

This reverts commit 47b5b441.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/17877#note_13488047

lib/support/nginx/gitlab

@@ -49,12 +49,7 @@ server {
 
     proxy_http_version 1.1;
 
-    ## By overwriting Host and clearing X-Forwarded-Host we ensure that
-    ## internal HTTP redirects generated by GitLab always send users to
-    ## YOUR_SERVER_FQDN.
-    proxy_set_header    Host                YOUR_SERVER_FQDN;
-    proxy_set_header    X-Forwarded-Host    "";
-
+    proxy_set_header    Host                $http_host;
     proxy_set_header    X-Real-IP           $remote_addr;
     proxy_set_header    X-Forwarded-For     $proxy_add_x_forwarded_for;
     proxy_set_header    X-Forwarded-Proto   $scheme;

lib/support/nginx/gitlab-ssl

@@ -93,12 +93,7 @@ server {
 
     proxy_http_version 1.1;
 
-    ## By overwriting Host and clearing X-Forwarded-Host we ensure that
-    ## internal HTTP redirects generated by GitLab always send users to
-    ## YOUR_SERVER_FQDN.
-    proxy_set_header    Host                YOUR_SERVER_FQDN;
-    proxy_set_header    X-Forwarded-Host    "";
-
+    proxy_set_header    Host                $http_host;
     proxy_set_header    X-Real-IP           $remote_addr;
     proxy_set_header    X-Forwarded-Ssl     on;
     proxy_set_header    X-Forwarded-For     $proxy_add_x_forwarded_for;