[libc++] Remove outdated setting of LIBCXX_CXX_ABI_LIBRARY_PATH (#100631)

LIBCXX_CXX_ABI_LIBRARY_PATH is used to determine the path of the ABI
library in use. It shouldn't be necessary to set it explicitly in order
to run the tests, since the test suite has its own configuration
mechanism.

Author: ldionne

libcxx/test/CMakeLists.txt

@@ -1,12 +1,6 @@
 include(HandleLitArguments)
 add_subdirectory(tools)
 
-# By default, libcxx and libcxxabi share a library directory.
-if (NOT LIBCXX_CXX_ABI_LIBRARY_PATH)
-  set(LIBCXX_CXX_ABI_LIBRARY_PATH "${LIBCXX_LIBRARY_DIR}" CACHE PATH
-      "The path to libc++abi library.")
-endif()
-
 set(AUTO_GEN_COMMENT "## Autogenerated by libcxx configuration.\n# Do not edit!")
 set(SERIALIZED_LIT_PARAMS "# Lit parameters serialized here for llvm-lit to pick them up\n")