Merge #21672: doc: remove boostrap info from GUIX_COMMON_FLAGS doc

09b3e46 doc: remove boostrap info from GUIX_COMMON_FLAGS doc (fanquake)

Pull request description:

  Passing `ADDITIONAL_GUIX_COMMON_FLAGS="--no-substitutes --bootstrap"` as suggested doesn't work:
  ```bash
        ...outputting in: '/bitcoin/guix-build-a1f0b8b62eb8/output/x86_64-linux-gnu'
            ...bind-mounted in container to: '/outdir-base/x86_64-linux-gnu'
  guix time-machine: error: bootstrap: unrecognized option
  ```

  and I think bootstrapping is more than covered in the preceding "Choose your security model" section.

ACKs for top commit:
  dongcarl:
    ACK 09b3e46
  jarolrod:
    ACK 09b3e46
  laanwj:
    Documentation review ACK 09b3e46

Tree-SHA512: e533a0b925a2ec091884ec04313f56376b4e85f615c8853dd51840181493d61bd01fee1c019e34880c32d7896e4871ea77e63398ba02b022f614c03bb16531aa

Author: fanquake

contrib/guix/README.md

@@ -205,10 +205,7 @@ find output/ -type f -print0 | sort -z | xargs -r0 sha256sum
 
 * _**ADDITIONAL_GUIX_COMMON_FLAGS**_
 
-  Additional flags to be passed to all `guix` commands. For a fully-bootstrapped
-  build, set this to `--bootstrap --no-substitutes` (refer to the [security
-  model section](#choosing-your-security-model) for more details). Note that a
-  fully-bootstrapped build will take quite a long time on the first run.
+  Additional flags to be passed to all `guix` commands.
 
 * _**ADDITIONAL_GUIX_TIMEMACHINE_FLAGS**_