[libcxx][AIX][z/OS] Remove headers included via `_IBMCPP__`

D127650 removed support for non-clang-based XL compilers, but left some
of the headers used only by this compiler and included under the
__IBMCPP__ macro. This change cleans this up by deleting these headers.

Reviewed By: hubert.reinterpretcast, fanbo-meng

Differential Revision: https://reviews.llvm.org/D129491