Remove code owners from low-risk files

This leaves an empty list, the behaviour of which isn't documented, but
experiments show github does not consider anyone to be a code owner
for those files and does not automatically request any reviews for them.

Author: Owen Jones

CODEOWNERS

@@ -48,12 +48,12 @@
 
 # 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