Drop check for LLVM_LIBCXX in FindHalide_LLVM.cmake (#8617)

This was never tested in CI, and grepping the LLVM sources (18 - main)
for this variable comes up empty. Mixing standard libraries in a single
project isn't a well-supported scenario anyway, so users wishing to
override the standard library should use a consistent toolchain file
between LLVM and Halide to do so.

Author: alexreinking

cmake/FindHalide_LLVM.cmake

@@ -115,11 +115,6 @@ if (Halide_LLVM_FOUND)
         set_property(TARGET Halide_LLVM::Core PROPERTY INTERFACE_CXX_RTTI "${LLVM_ENABLE_RTTI}")
         set_property(TARGET Halide_LLVM::Core APPEND PROPERTY COMPATIBLE_INTERFACE_BOOL CXX_RTTI)
 
-        if (LLVM_LIBCXX GREATER -1)
-            target_compile_options(Halide_LLVM::Core INTERFACE "$<$<LINK_LANGUAGE:CXX>:-stdlib=libc++>")
-            target_link_options(Halide_LLVM::Core INTERFACE "$<$<LINK_LANGUAGE:CXX>:-stdlib=libc++>")
-        endif ()
-
         if (Halide_LLVM_SHARED_LIBS)
             target_link_libraries(Halide_LLVM::Core INTERFACE LLVM ${CMAKE_DL_LIBS})
         else ()