shelve: drop unnecessary backup of narrowspec
I mechanically added the backup code everywhere in
ad24b581e4d9
(narrow: call narrowspec.{save,restore,clear}backup directly,
2018-08-03), but I can't think of a reason it would be needed in the
shelve code, so let's drop it.
Differential Revision: https://phab.mercurial-scm.org/D5457
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