encoding: remove unnecessary lambdas from _encodingfixers
They have been unnecessary since 1a3a08b5d4d5 (encoding: remove
workaround for locale.getpreferredencoding(), 2017-05-13). Also rename
the variable since "fixer" sounds like a function.
Differential Revision: https://phab.mercurial-scm.org/D4743
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state