nvk: Clean up unused header from libdrm_nouveau

This was added in 6e008930 without a dependency on libdrm_nouveau. If libdrm is not compiled with nouveau enabled, the build errors here.

This is currently unused and since 821f4c8d removed dependency on libdrm_nouveau, this should be gone too.

Fixes: 821f4c8d ("nouveau: import libdrm_nouveau")