[CMake] Don't explicitly use LLVM_LIBRARY_DIR in standalone builds

Summary:
This line is unnecessary because add_llvm_executable will handle
linking the correct LLVM libraries for you. LLDB standalone builds are totally
fine without this.

In the best case, having this line here is harmless. In the worst case it can
cause link issues.

If you build lldb-server for android using the standalone build, this line
will cause LLVM_LIBRARY_DIR to be the first place you look for libraries.
This is an issue because if you built libc++, it will try to link against
that one instead of the one from the android NDK.  Meanwhile, the LLVM libraries
you're linking against were linked against the libc++ from the NDK.

Ideally, we would take advantage of the AFTER option for link_directories(), but
that was not available in LLDB's minimum supported version of CMake (CMake 3.4.3).

Differential Revision: https://reviews.llvm.org/D60180

llvm-svn: 357817