Remove option-based emacsy key bindings

[lishid]

Fixes #6630

Removes Alt-D, Alt-F and Alt-B from the default emacsy key bindings, which is enabled by default on macOS.

A PR that moves those from emacsy to the proper emacs map in keymap/emacs.js would be very welcome.

It seems like these three are already implemented in keymap/emacs.js

CodeMirror/keymap/emacs.js

Line 309 in 8bc57f7

"Alt-F": move(byWord, 1), "Alt-B": move(byWord, -1),

CodeMirror/keymap/emacs.js

Line 311 in 8bc57f7

"Alt-D": function(cm) { killTo(cm, byWord, 1, "grow"); },

[marijnh]

Thank you!