Revert "Errors with the same span print the span once"

This reverts commit 6030e3982a52c8d9ede225f992088bb75fba4ef1.
auto merge of #6017 : brson/rust/revert-span-merge, r=catamorphism

This reverts commit 6030e3982a52c8d9ede225f992088bb75fba4ef1.

This reorders error messages in ways that aren't intended. A more satisfying solution will require an interface that allows diagnostics to be grouped together, so that messages that logically belong together aren't reordered.

#4569