Remove obsolete clear_db management command.

do_destroy_rebuild_database is the "new" way to reset one's dev
database.

(imported from commit 8edb707ecd5e6e24b62eef324915cc3a4a5b8eda)

zerver/management/commands/clear_db.py

@@ -1,11 +0,0 @@
-from __future__ import absolute_import
-
-from django.core.management.base import NoArgsCommand
-from zerver.models import clear_database
-
-class Command(NoArgsCommand):
-    help = "Clear only tables we change: messages, accounts + sessions"
-
-    def handle_noargs(self, **options):
-        clear_database()
-        self.stdout.write("Successfully cleared the database.\n")