intel: Drop legacy execbuffer support

Execbuffer2 support was introduced to libdrm in b5096402, 10 years
ago, and no driver has used the old execbuf path since.  There's no need
to support 10-year-old kernels.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
6 jobs for main in 43 seconds (queued for 5 seconds)
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passed debian #14032598

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passed meson-armhf #14032602

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passed meson-i386 #14032600

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passed meson-ppc64el #14032603

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passed meson-x86_64 #14032599

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