Disable unused about: pages

There are several about: pages listed in browser/components/about/AboutRedirector.cpp and in docshell/base/nsAboutRedirector.cpp.

We should disable several of them. Some don't work because the file is removed at compile time (e.g., about:firefoxview), others are still displayed (about:pocket-*), but non-functional.

We could comment a few of them.