Remove system-*.txt files

Nothing depends on these anymore. The same data is available through
the build artifact framework-base-api-system-current.txt.

Bug: 171029603
Test: m checkbuild
Change-Id: I8d5f0a132aa307d88fbc2bd7521cdd4bdd8c6cee
Merged-In: I8d5f0a132aa307d88fbc2bd7521cdd4bdd8c6cee
(cherry picked from commit d5cba345)