Drop "dissimilar" feature, keeping only "diff"

Previously, a dependency `trybuild = { features = ["dissimilar"] }`
would compile (and silently have no effect). The correct usage is
`trybuild = { features = ["diff"] }` and is independent of the name of
the underlying library being used for the diff computations.

Cargo.toml

@@ -16,7 +16,7 @@ rust-version = "1.70"
 # Experimental: highlight the diff between the expected and actual compiler
 # output. Currently unix-only. If you test this out, please provide any feedback
 # in https://github.com/dtolnay/trybuild/issues/41.
-diff = ["dissimilar"]
+diff = ["dep:dissimilar"]
 
 [dependencies]
 dissimilar = { version = "1.0", optional = true }