[libc++][test] Use = delete over DELETE_FUNCTION. NFC.

Some tests repeat the definition of `DELETE_FUNCTION` macro locally.
However, it's not even requred to guard against in the C++03 case since
Clang supports `= delete;` in C++03 mode. A warning is issued but
`libc++` tests run with `-Wno-c++11-extensions`, so this isn't an issue.
Since we don't support other compilers in C++03 mode, `= delete;` is
always available for use. As such, inline all calls of `DELETE_FUNCTION`
to use `= delete;`.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D111148