simplemerge: remove code for checking binary input now that callers do it
The callers now do the checking for binary inputs and handle warnings
and/or errors, so we can remove that code from the low-level
`simplemerge` module now. After this patch we just raise an error
unless the caller told us to allow binary inputs.
Differential Revision: https://phab.mercurial-scm.org/D12169
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