[lldb] Cleans up system_libs

Summary:
Long long ago system_libs was appended to LLDB_SYSTEM_LIBS in
cmake/LLDBDependencies.cmake. After that file was removed, system_libs
is orphaned.

Currently the only user is source/Utility. Move the logic there and
remove system_libs.

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D80253