Remove Ctrl-f/b from the default keymap

[drybalka]

The main reason for this proposal is that Ctrl-u/d already provide the scrolling functionality anyone would need, they are consistent across different modes, mnemonic, easy to memorize both for beginners and vim-gurus, and easily provide the same "page_up/down" motion.

The other arguments in favor of removing Ctrl-f/b from defaults are:

• Simplify cognitive load on a user - scrolling is always done by Ctrl-u/d, period.
• Ctrl-f/b does not work for scrolling in popups, which is confusing
• Having 2 sets of scrolling keymaps is confusing for beginners
• "page_up/down" is already mapped to physical PageUp/Down keys
• The default keymap is huge as it is (and quite intimidating for beginners), there is no need in making it larger
• Allow a more useful behavior for such a convenient keymaps
• I would argue, Ctrl-f/b is mostly used by veteran vim users, who tweak their config anyway and can easily restore the previous behavior

This issue somewhat relates to #1614

[nik-rev]

I often use Ctrl-f/b to scroll pages fast when I am not "looking for something", so I am opposed to this