Adjust list of compression methods (#655)

According to https://github.com/armbian/build/blob/main/lib/functions/image/compress-checksum.sh

Author: EvilOlaf

docs/Developer-Guide_Build-Switches.md

@@ -315,10 +315,7 @@ Enforce building without Armbian repository. Suitable for developing new release
 - **COMPRESS_OUTPUTIMAGE** (comma-separated list): create a compressed archive with an image file and GPG signature for redistribution
   - sha: generate SHA256 hash for image
   - gpg: sign image using gpg
-  - 7z: compress image, hash, and signature to 7z archive
-  - gz: compress image only using gz format
   - xz: compress image only using xz format
-  - yes: compatibility shortcut for `sha,gpg,7z`
 - **IMAGE_XZ_COMPRESSION_RATIO** ( **1** - 9 ) images compression levels when using xz compressor. Beware of memory consumption when going higher
 - **SEVENZIP** ( yes | **no** ): create .7z archive with extreme compression ratio instead of .zip
 - **BUILD_KSRC** ( **yes** | no ): create kernel source packages while building...