[CMake] Remove redundant logic in runtimes/CMakeList.txt

Summary:
`SUB_CHECK_TARGETS` contains all test targets in `SUB_COMPONENTS` when
we load `Components.cmake`. We don't need to add those targets
again and having duplicate targets will break the cmake policy CMP0002.

Reviewers: phosek

Subscribers: mgorny, llvm-commits, srhines, pirama

Differential Revision: https://reviews.llvm.org/D35692

llvm-svn: 308900