Revert f8990feb "[libclang] Install both libclang.a and libclang.so when...

Revert f8990feb "[libclang] Install both libclang.a and libclang.so when LIBCLANG_BUILD_STATIC=ON"

This broke builds configured with

$ cmake -GNinja -DCMAKE_BUILD_TYPE=Release '-DLLVM_ENABLE_PROJECTS=clang' '-DLLVM_TARGETS_TO_BUILD=X86' -DLLVM_ENABLE_PIC=OFF ../llvm

CMake Error at
/b/s/w/ir/cache/builder/src/third_party/llvm/clang/tools/libclang/CMakeLists.txt:123
(target_compile_definitions):
    target_compile_definitions called with non-compilable target type

This reverts commit f8990feb.