Summary:
Add support to hg book -d to delete scratch infinitepush bookmarks
Uses functions from remotenames to rewrite the remotenames cache omitting
the specified scratch bookmarks
Test Plan:
cd ~/facebook-hg-rpms/fb-hgext/tests
source ../../hg-dev
rt test-infinitepush-*.t --extra-config-opt=devel.all-warnings=False
Reviewers: #mercurial, cdown, stash, durham
Differential Revision: https://phabricator.intern.facebook.com/D6221853
Tasks: T22615396
