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
local test passed
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