Revert "core: can't add boxes from network shares [GH-3279]"

This reverts commit 41636a8.

Conflicts:
	CHANGELOG.md

lib/vagrant/action/builtin/box_add.rb

@@ -41,12 +41,6 @@ def call(env)
             # Expand the path and try to use that, if possible
             p = File.expand_path(URI.unescape(u.gsub(/^file:\/\//, "")))
             p = Util::Platform.cygwin_windows_path(p)
-
-            # If this is a network share on Windows then show the user an error
-            if Util::Platform.windows? && (p.start_with?("//") || p.start_with?("\\\\"))
-              raise Errors::BoxUrlIsNetworkShare, url: p
-            end
-
             next "file://#{URI.escape(p.gsub("\\", "/"))}" if File.file?(p)
 
             u

lib/vagrant/errors.rb

@@ -236,10 +236,6 @@ class BoxUpdateNoMetadata < VagrantError
       error_key(:box_update_no_metadata)
     end
 
-    class BoxUrlIsNetworkShare < VagrantError
-      error_key(:box_url_is_network_share)
-    end
-
     class BoxVerificationFailed < VagrantError
       error_key(:failed, "vagrant.actions.box.verify")
     end

templates/locales/en.yml

@@ -545,13 +545,6 @@ en:
         directly instead of from a box catalog. Vagrant can only
         check the versions of boxes that were added from a catalog
         such as from the public Vagrant Server.
-      box_url_is_network_share: |-
-        You specified a network share path as a path to add a box. Vagrant
-        current doesn't support network share URLs. Please copy the file
-        from the network share to your local machine, then add it from
-        there.
-
-        URL: %{url}
       bundler_disabled: |-
         Vagrant's built-in bundler management mechanism is disabled because
         Vagrant is running in an external bundler environment. In these