Remove the unused -Z trans-time-graph flag.

Rebase of rust-lang#50783 has accidentally revived the flag (which should be
renamed to `-Z codegen-time-graph` by rust-lang#50615).

src/librustc/session/config.rs

@@ -1304,8 +1304,6 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "enable polonius-based borrow-checker"),
     codegen_time_graph: bool = (false, parse_bool, [UNTRACKED],
         "generate a graphical HTML report of time spent in codegen and LLVM"),
-    trans_time_graph: bool = (false, parse_bool, [UNTRACKED],
-        "generate a graphical HTML report of time spent in trans and LLVM"),
     thinlto: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "enable ThinLTO when possible"),
     inline_in_all_cgus: Option<bool> = (None, parse_opt_bool, [TRACKED],