This message is displayed in the Eclipse error log after removing the REST model extension definition (MED) from a 7.4 REST-enabled model:
Value " " is not an allowed value for property "ext-custom:REST-METHOD." Value was reset to be an empty value.
The message is being logged from ModelExtensionPropertyDescriptor (line 249). It is only a log message and doesn't really affect anything but should be fixed. The properties are successfully removed from the model.