Disable whole project indexing instead of disabling clangd when running on lower specs computers

Starting with https://codereview.qt-project.org/c/qt-creator/qt-creator/+/416982 clangd will be disabled if the user has less than 12 GB of RAM.

Users that had Qt Creator versions and having everything fine with the libclang parser would get a subpar experience starting with Qt Creator 8.

By disabling the whole project indexing, which is called "background indexing" in settings, the users will get the same experience as with the libclang parser.