[libcxxabi] Remove unnecessary define from build

Now that we're passing -D_LIBCPP_BUILDING_LIBRARY to the libc++abi
build, -D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS is redundant
(https://github.com/llvm/llvm-project/blob/fb3a00c327df78eaa534e53ac6f07112e0585121/libcxx/include/exception#L120-L121
is the only use of _LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS in
libc++, and that conditional also checks for _LIBCPP_BUILDING_LIBRARY).

Reviewed By: #libc_abi, phosek

Differential Revision: https://reviews.llvm.org/D102372