Martin von Zweigbergk <martinvonz@google.com> [Mon, 11 Jan 2021 23:08:37 -0800] rev 46295
shelve: add a method for deleting shelf to new shelf class
This is not necessary for my future changes, but it's more consistent
to encapsulate the knowledge of the various files in the `Shelf`
class.
Differential Revision: https://phab.mercurial-scm.org/D9742