fix: remove the 'fileBasedReporters' property from the 'stats' object

BREAKING CHANGE: The 'fileBasedReporters' property has been in the 'stats' object by accident and now is removed. Apparently it has been there for a while, as it got listed in #599, the only documentation of the 'stats' object to this date.

lib/reporters/BaseReporter.js

@@ -16,6 +16,14 @@ BaseReporter.prototype.configureEmitter = function configureEmitter(emitter) {
   emitter.on('end', (callback) => {
     this.stats.end = new Date();
     this.stats.duration = this.stats.end - this.stats.start;
+
+    // FIXME: It looks like 'configureReporters' sneaks in this property to let
+    // the reporters know how many files they should expect. This obviously
+    // isn't ideal interface and it pollutes the 'stats' object, which is
+    // a part of Dredd's public interface. This line cleans it up for now, but
+    // ideally reporters would learn about the number of files in a nicer way.
+    delete this.stats.fileBasedReporters;
+
     callback();
   });