doc: remove CII badge in README
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc
I agree with @trott in https://github.com/nodejs/node/pull/18884#pullrequestreview-97959453, this badge servers a questionable purpose and I think we can safely remove it.