Remove -Z force-overflow-checks

It was replaced several years ago by the stable option `-C
overflow-checks`. The goal was to delete the `-Z` flag once users had
migrated [1]. Now that it's been several years, it makes sense to delete
the old flag.

See also the discussion on Zulip [2].

[1]: https://github.com/rust-lang/rust/issues/33134#issuecomment-280484097
[2]: 262497224

compiler/rustc_session/src/options.rs

@@ -1131,8 +1131,6 @@ options! {
     fewer_names: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "reduce memory use by retaining fewer names within compilation artifacts (LLVM-IR) \
         (default: no)"),
-    force_overflow_checks: Option<bool> = (None, parse_opt_bool, [TRACKED],
-        "force overflow checks on or off"),
     force_unstable_if_unmarked: bool = (false, parse_bool, [TRACKED],
         "force all crates to be `rustc_private` unstable (default: no)"),
     fuel: Option<(String, u64)> = (None, parse_optimization_fuel, [TRACKED],