docs/env_vars: drop WLR_DIRECT_TTY

The direct session is gone, so this env var isn't looked up
anymore.

docs/env_vars.md

@@ -6,7 +6,6 @@ wlroots reads these environment variables
   libinput, drm, wayland, x11, headless, noop)
 * *WLR_NO_HARDWARE_CURSORS*: set to 1 to use software cursors instead of
   hardware cursors
-* *WLR_DIRECT_TTY*: specifies the tty to be used (instead of using /dev/tty)
 * *WLR_XWAYLAND*: specifies the path to an Xwayland binary to be used (instead
   of following shell search semantics for "Xwayland")
 * *WLR_RENDERER*: forces the creation of a specified renderer (available