chore: remove *.orig files from version control

By git mergetool docs:
> git mergetool creates *.orig backup files while resolving merges. These
> are safe to remove once a file has been merged and its git mergetool
> session has completed.

As there were no references found to these *.orig files in the codebase,
the assumption is that these files have been included accidentally by
executing some variation of `git add secp256k1-sys/depend/secp256k1/*`,
and they seem to not serve any use case, therefore they will be removed
from version control by this commit.