Remove support for -Zcombine-cgu

Nobody seems to actually use this, while still adding some extra
complexity to the already rather complex codegen coordinator code.
It is also not supported by any backend other than the LLVM backend.

compiler/rustc_session/src/options.rs

@@ -2168,8 +2168,6 @@ options! {
         "hash algorithm of source files used to check freshness in cargo (`blake3` or `sha256`)"),
     codegen_backend: Option<String> = (None, parse_opt_string, [TRACKED],
         "the backend to use"),
-    combine_cgu: bool = (false, parse_bool, [TRACKED],
-        "combine CGUs into a single one"),
     contract_checks: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "emit runtime checks for contract pre- and post-conditions (default: no)"),
     coverage_options: CoverageOptions = (CoverageOptions::default(), parse_coverage_options, [TRACKED],