mesa: Replace gl_extensions::EXT_texture3D with ::dummy_true

mesa: Replace gl_extensions::EXT_texture3D with ::dummy_true

Mesa unconditionally sets this driver flag to true in
_mesa_init_extensions(). There is therefore no need for
the driver to communicate support for this extension.
Replace the driver capability flag with ::dummy_true.

Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>