Don't import subprojects from landscape

[cjyabraham]

Projects like Hubble are subprojects and should not be imported into CNCF.io as proper projects. Getting rid of Hubble should also resolve the missing logo here.

This is currently blocked by this issue

[cjyabraham]

Subprojects that need to go away:

• Hubble
• Flagger
• Tetragon

[cjyabraham]

The landscape.yml file has been updated to remove the project maturity level attributes. With these attributes missing, the subprojects listed above get skipped so should not be imported anymore. Subsequent subprojects added to landscape should make sure they don't have the project attribute present or else they will be included and will need to be manually deleted from cncf.io.