hardening: drop obsolete DRM_LEGACY from config fragment

hardening: drop obsolete DRM_LEGACY from config fragment

Commit 94f8f319cbcb ("drm: Remove Kconfig option for legacy support
(CONFIG_DRM_LEGACY)") removes the config DRM_LEGACY, but one reference to
that config is left in the hardening.config fragment.

As there is no drm legacy driver left, we do not need to recommend this
attack surface reduction anymore.

Drop this reference in hardening.config fragment.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20240208091045.9219-3-lukas.bulwahn@gmail.com
Signed-off-by: Kees Cook <keescook@chromium.org>