gallium: Remove the malloc pipebuffer manager.

This has been unused since r600 stopped using it in 2010.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Kristian Høgsberg <hoegsberg@gmail.com>
14 jobs for !707 with gallium-cleanup in 13 minutes and 19 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Containers Build
passed debian #257780

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  Build+Test
passed meson-clang #257782

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passed meson-gallium-clover-llvm #257788

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passed meson-gallium-clover-llvm39 #257789

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passed meson-gallium-drivers-other #257787

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passed meson-gallium-radeonsi #257786

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passed meson-gallium-st-other #257790

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passed meson-gallium-swr #257781

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passed meson-glvnd #257785

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passed meson-loader-classic-dri #257784

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passed meson-vulkan #257783

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passed scons-llvm #257792

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passed scons-nollvm #257791

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passed scons-swr #257793

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