Simplify platforms table

Author: dtolnay

README.md

@@ -54,16 +54,16 @@ $ buck2 build :rustc_ast[profile][llvm_passes] --show-output
 
 The following platforms are available for use with `--target-platforms`:
 
-| target platform | uses rustc, rustdoc, clippy | uses standard library |
-|---|---|---|
-| `//platforms/stage1:compiler` | downloaded stage0 | downloaded stage0 |
-| `//platforms/stage1:compiler-build-script` | downloaded stage0 | downloaded stage0 |
-| `//platforms/stage1:library` | built-from-source stage1 | none |
-| `//platforms/stage1:library-build-script` | downloaded stage0 | downloaded stage0 |
-| `//platforms/stage2:compiler` | built-from-source stage1 | built-from-source stage1 |
-| `//platforms/stage2:compiler-build-script` | built-from-source stage1 | built-from-source stage1 |
-| `//platforms/stage2:library` | built-from-source stage2 | none |
-| `//platforms/stage2:library-build-script` | built-from-source stage1 | built-from-source stage1 |
+| target platform | uses rustc, rustdoc, clippy |
+|---|---|
+| `//platforms/stage1:compiler-build-script` | downloaded stage0 |
+| `//platforms/stage1:compiler` | downloaded stage0 |
+| `//platforms/stage1:library-build-script` | downloaded stage0 |
+| `//platforms/stage1:library` | built-from-source stage1 |
+| `//platforms/stage2:compiler-build-script` | built-from-source stage1 |
+| `//platforms/stage2:compiler` | built-from-source stage1 |
+| `//platforms/stage2:library-build-script` | built-from-source stage1 |
+| `//platforms/stage2:library` | built-from-source stage2 |
 
 The "build-script" platforms compile without optimization. These are used for
 procedural macros and build.rs. The non-build-script platforms compile with a