localrepo: remove unused requirements attributes on localpeer (API)
The previous changeset removed the last consumer of requirements. I'm
not sure when supportedformats became unused. But I couldn't find
any obvious instances where it is being used. It likely stems from
peers being derived from repository instances several years ago and
is a holdover from that day.
Differential Revision: https://phab.mercurial-scm.org/D266
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