keyboard: Drop "Left Alt" as compose key option

One of the compose key options in Settings -> Keyboard (actually the first one in the list) is "Left Alt". However, that option does not exist in XKB:

$ cat /usr/share/X11/xkb/rules/evdev.lst | grep compose:
  compose:ralt         Right Alt
  compose:lwin         Left Win
  compose:lwin-altgr   3rd level of Left Win
  compose:rwin         Right Win
  compose:rwin-altgr   3rd level of Right Win
  compose:menu         Menu
  compose:menu-altgr   3rd level of Menu
  compose:lctrl        Left Ctrl
  compose:lctrl-altgr  3rd level of Left Ctrl
  compose:rctrl        Right Ctrl
  compose:rctrl-altgr  3rd level of Right Ctrl
  compose:caps         Caps Lock
  compose:caps-altgr   3rd level of Caps Lock
  compose:102          The "&lt; &gt;" key
  compose:102-altgr    3rd level of the "&lt; &gt;" key
  compose:paus         Pause
  compose:ins          Insert
  compose:prsc         PrtSc
  compose:sclk         Scroll Lock

Consequently, if you select "Left Alt" as your compose key, it won't work, so that option should be dropped.