Replace crashreport admin management classes with register calls

ModelAdmin classes are not needed if no custom values are defined:
https://docs.djangoproject.com/en/1.11/ref/contrib/admin/#modeladmin-objects

Issue: HIC-195
Change-Id: I9838c01f3d7f9deeff82e63545783a5c87eab164
diff --git a/crashreports/admin.py b/crashreports/admin.py
index c45eabd..33f646e 100644
--- a/crashreports/admin.py
+++ b/crashreports/admin.py
@@ -6,30 +6,7 @@
 from crashreports.models import HeartBeat
 from crashreports.models import LogFile
 
-
-@admin.register(Crashreport)
-class CrashreportAdmin(admin.ModelAdmin):
-    """Manage Crashreports as admin user."""
-
-    pass
-
-
-@admin.register(HeartBeat)
-class CrashreportAdmin(admin.ModelAdmin):
-    """Manage HeartBeats as admin user."""
-
-    pass
-
-
-@admin.register(LogFile)
-class CrashreportAdmin(admin.ModelAdmin):
-    """Manage LogFiles as admin user."""
-
-    pass
-
-
-@admin.register(Device)
-class CrashreportAdmin(admin.ModelAdmin):
-    """Manage Devices as admin user."""
-
-    pass
+admin.site.register(Crashreport)
+admin.site.register(HeartBeat)
+admin.site.register(LogFile)
+admin.site.register(Device)