Don't use cbmc-developers as a code owner

The GitHub documentation says that in CODEOWNERS you should only
specify people with write access. @diffblue/cbmc-developers does not
have write access. We want to keep the current behaviour that no one is
assigned as code owner, so we leave the list blank instead.

Author: Owen

CODEOWNERS

@@ -1,6 +1,3 @@
-# These owners will be the default owners for everything in the repo.
-*       @kroening @tautschnig @peterschrammel @smowton @chrisr-diffblue
-
 # These files should rarely change
 
 src/big-int/ @kroening
@@ -48,13 +45,6 @@ src/cpp/ @kroening @tautschnig @peterschrammel
 
 # These files change frequently and changes are low-risk
 
-src/util/irep_ids.def @diffblue/cbmc-developers
-
-unit/ @diffblue/cbmc-developers
-regression/ @diffblue/cbmc-developers
-jbmc/unit/ @diffblue/cbmc-developers
-jbmc/regression/ @diffblue/cbmc-developers
-
 scripts/ @diffblue/devops @thk123 @forejtv @peterschrammel
 .travis.yml @diffblue/devops @thk123 @forejtv @peterschrammel
 appveyor.yml @diffblue/devops @thk123 @forejtv @peterschrammel