docs: remove leftover mention of meson dri3 option

Remove some leftover mention of the meson dri3 option from the
documentation, the option has been removed in commit 8f6fca8 (meson:
delete dri3 build option, 2024-08-30)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32251>

Author: ao2

docs/gallium-nine.rst

@@ -24,7 +24,6 @@ Beware: Most Direct3D games are 32-bit, and thus need a 32-bit version of Mesa.
 
    $ meson configure \
          -D gallium-nine=true \
-         -D dri3=enabled \
          ...
 
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