Remove dependency on crate source-span (#49)

This dependency was listed in crates/oq3_source_file/Cargo.toml
But in fact it is not used. `source-span` depends on `termion`.
The latter does not work on Windows, so we don't want it.

It may be possible to disable the `termion` feature if for some reason
`source-span` turns out to be useful.

crates/oq3_source_file/Cargo.toml

@@ -17,4 +17,3 @@ doctest = false
 [dependencies]
 ariadne = { version = "0.3.0", features = ["auto-color"] }
 oq3_syntax.workspace = true
-source-span = { version = "2.7.0" }