histedit: drop --no-backup option
Dropping this option because now we have a better option
than passing --no-backup flag every time, now user can
set a config in hgrc:
[ui]
history-editing-backup = False
This config aims to operate on every history editing command and
it is still work in progress. As yuya suggessted it probably to
late to add full support this config, so making this as
an experimental config.
Differential Revision: https://phab.mercurial-scm.org/D3965
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