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. So hopefully this PR will not change things.

Author: Owen Jones

CODEOWNERS

@@ -48,12 +48,12 @@ src/cpp/ @kroening @tautschnig @peterschrammel
 
 # These files change frequently and changes are low-risk
 
-src/util/irep_ids.def @diffblue/cbmc-developers
+src/util/irep_ids.def
 
-unit/ @diffblue/cbmc-developers
-regression/ @diffblue/cbmc-developers
-jbmc/unit/ @diffblue/cbmc-developers
-jbmc/regression/ @diffblue/cbmc-developers
+unit/
+regression/
+jbmc/unit/
+jbmc/regression/
 
 scripts/ @diffblue/devops @thk123 @forejtv @peterschrammel
 .travis.yml @diffblue/devops @thk123 @forejtv @peterschrammel