Remove support for #[rustc_mir(borrowck_graphviz_format="gen_kill")].

Because it's equivalent to `#[rustc_mir(borrowck_graphviz_format)]`. It
used to be distinct, but the distinction was removed in
rust-lang@3233fb1.

Author: nnethercote

Diff for: compiler/rustc_mir_dataflow/src/framework/graphviz.rs

@@ -122,7 +122,7 @@ impl RustcMirAttrs {
                 })
             } else if attr.has_name(sym::borrowck_graphviz_format) {
                 Self::set_field(&mut ret.formatter, tcx, &attr, |s| match s {
-                    sym::gen_kill | sym::two_phase => Ok(s),
+                    sym::two_phase => Ok(s),
                     _ => {
                         tcx.dcx().emit_err(UnknownFormatter { span: attr.span() });
                         Err(())

Diff for: compiler/rustc_span/src/symbol.rs

@@ -1068,7 +1068,6 @@ symbols! {
         ge,
         gen_blocks,
         gen_future,
-        gen_kill,
         generator_clone,
         generators,
         generic_arg_infer,