Merge pull request #8177 from jboeuf/remove_call_creds_from_channel_creds

...ib/security/credentials/composite/composite_credentials.c

...ib/security/credentials/composite/composite_credentials.h

src/core/lib/security/credentials/credentials.c

src/core/lib/security/credentials/credentials.h

src/core/lib/security/credentials/fake/fake_credentials.c

src/core/lib/security/credentials/ssl/ssl_credentials.c

test/core/security/credentials_test.c