remove non-target entries leaking into DawnTargets.cmake

[domibel]

I am using find_package(Dawn REQUIRED) and got:

CMake Error: The link interface of target "dawn::webgpu_dawn" contains:
  dawn::partition_alloc
but the target was not found.

DawnTargets.cmake incorrectly includes dawn::partition_alloc
since commit 96e9e61 as part of webgpu_dawn's link interface:

set_target_properties(dawn::webgpu_dawn PROPERTIES
  INTERFACE_LINK_LIBRARIES "...;dawn::partition_alloc;...")

dawn::partition_alloc is a header-only internal target that is never
installed, so any downstream find_package(Dawn) fails.

[github-actions]

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[kainino0x]

@meuter you added this code in #60, does this change break things for you?

[meuter]

Hi @kainino0x, I just double checked and the Android monolithic build is still working even with the patch in this PR. liblog is collected in the other loop here . I guess I was a little too eager when collecting dependencies. I assumed wrongly from this comment that those were public interface targets.

are there any public interface (header only) targets that could be missed with this PR?