Make RemoteMirror's only_protected_branches default value consistent

From https://gitlab.com/gitlab-org/gitlab-ce/issues/53515, we see the
backend appears to have inconsistent default values for this column:

* DB schema: false by default
* UI checkbox: false by default
* `RemoteMirror` model: true by default

This leads to unintended behavior where the boolean is activated if the
UI doesn't pass in a value for the checkbox.

app/models/remote_mirror.rb

@@ -15,8 +15,6 @@ class RemoteMirror < ActiveRecord::Base
                  insecure_mode: true,
                  algorithm: 'aes-256-cbc'
 
-  default_value_for :only_protected_branches, true
-
   belongs_to :project, inverse_of: :remote_mirrors
 
   validates :url, presence: true, url: { protocols: %w(ssh git http https), allow_blank: true, enforce_user: true }

spec/models/remote_mirror_spec.rb

@@ -174,7 +174,15 @@ describe RemoteMirror do
     end
 
     context 'with remote mirroring enabled' do
+      it 'defaults to disabling only protected branches' do
+        expect(remote_mirror.only_protected_branches?).to be_falsey
+      end
+
       context 'with only protected branches enabled' do
+        before do
+          remote_mirror.only_protected_branches = true
+        end
+
         context 'when it did not update in the last minute' do
           it 'schedules a RepositoryUpdateRemoteMirrorWorker to run now' do
             expect(RepositoryUpdateRemoteMirrorWorker).to receive(:perform_async).with(remote_mirror.id, Time.now)