header-checker: Remove obsolete merge-abi-diff

This commit removes obsolete merge-abi-diff.  This command hasn't been
used for a while (not referenced by the build system).

Test: OUT_DIR=out \
      development/vndk/tools/header-checker/android/build-prebuilts.sh
Change-Id: I2de5189cca6354183f2bb65f6cc652251a32094b
3 files changed