xlog: introduce xlog_remove_file function for removing xlog files

This commit introduces the xlog_remove_file() function that removes
a file by name and logs the error on failure. We use this function
everywhere we delete xlog files so that there's a single place where we
call unlink(). We also factor out the core functionality to a callback
function that can be overridden. This will help us implement thorough
file deletion.

Needed for tarantool/tarantool-ee#540

NO_DOC=refactoring
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