Sunsetting Shields custom logos

Following discussions in #9476, we’ve gone ahead and deleted all custom logos that were maintained on the Shields.io side (bitcoin, dependabot, gitlab, npm, paypal, serverfault, stackexchange, superuser, telegram, travis), and will solely rely on the Simple-Icons project to provide named logos for our badges from now on. If you were using a Shields custom logo, you will have transparently switched over to the corresponding Simple-Icon and do not need to make changes to your badges.