refactor: remove internal fn not needed any more
refactor: remove internal fn not needed any more

The function was needed to copy region information from value to value,
for a useful error message. Because of the last few commits, the region
information about keys are passed to insert_nested_keys that requires
the function which is removed. And it turned out that the function is no
longer required. It is originally a workaround, so I removed it.

ToruNiina/toml11