HI Augustin,
The new UX is deployed as part of .WAR so below is the steps
1. Copy the PM.WAR from C:\PPM_HOME\tomcat-app-deploy\webapps
2. Keep a back up and extract the PM.WAR using 7zip
3. Once extracted search for login.html and it should be like in windows C:\pm\js\core\login
4. Comment out the below lines
<div class="col-md-6" style="margin-right:0px; text-align: right;" >
<a href="#changepassword" ng-if="isForgetPasswordLinkEnabled()" class="ppm-forgotpass-link" id="forgotpassword">
<span translate="login.form.forgotpassword">Forgot Password</span>
</a>
</div>
5. Package the PM.war and place it under C:\PPM_HOME\tomcat-app-deploy\webapps
Doing this you will not see the Forgot password link.
Disclaimer: This is not something CA Suggest you to do but as you are looking for customization, i have recommended the same.
Regards
Suman Pramanik