xtask/miri: remove flag that is enabled by default

Author: phip1611

xtask/src/cargo.rs

@@ -201,7 +201,6 @@ impl Cargo {
                 }
             }
             CargoAction::Miri => {
-                cmd.env("MIRIFLAGS", "-Zmiri-tag-raw-pointers");
                 action = "miri";
                 sub_action = Some("test");
             }