Remove TARGET_VNDK_USE_CORE_VARIANT

We introduced this in
https://android-review.googlesource.com/c/platform/build/+/1087517
but now with VNDK deprecated, this is no longer in use so we
remove it for clarity.

Bug: 319529309
Test:  TreeHugger skipped running mokey_go32 boot test because there were no changes in the artifacts.  Additionally, built a Go device, booted, and /linkerconfig/ld.config.txt is the same and things run as normal.
Change-Id: Ifba30d7cc30c641f265abe52ed2cf7c02660a5ec