Meta+CMake: Remove "image" ninja target in favor of "qemu-image"

"image" was an alias for "qemu-image".

I want to add an `image` userland utility, which clashes with that
shortname.

So remove the existing "image" target. It was just an alias for
"qemu-image".

If you use serenity.sh to build, nothing changes. This only affects you
if you run ninja manually -- you now have to say `ninja qemu-image` to
build the disk image.

Meta/Azure/Serenity.yml

@@ -65,7 +65,7 @@ jobs:
         CCACHE_DIR: '$(SERENITY_CCACHE_DIR)'
 
     - script: |
-        ninja install && ninja image
+        ninja install && ninja qemu-image
       displayName: 'Create RootFS'
       workingDirectory: $(Build.SourcesDirectory)/Build/${{ parameters.arch }}clang