Fix oversight: remove __NVCOMPILER elif branch in PYBIND11_BUILD_ABI block.

Also add comment pointing to this PR (#5439).

Author: rwgk

include/pybind11/conduit/pybind11_platform_abi_id.h

@@ -64,11 +64,9 @@
 #                error "Unknown major version for MSC_VER: PLEASE REVISE THIS CODE."
 #            endif
 #        endif
-#    elif defined(__NVCOMPILER)        // NVHPC (PGI-based).
-#        define PYBIND11_BUILD_ABI ""  // TODO: What should be here, to prevent UB?
 #    elif defined(_LIBCPP_ABI_VERSION) // https://libcxx.llvm.org/DesignDocs/ABIVersioning.html
 #        define PYBIND11_BUILD_ABI "_abi" PYBIND11_PLATFORM_ABI_ID_TOSTRING(_LIBCPP_ABI_VERSION)
-#    elif defined(__GXX_ABI_VERSION)
+#    elif defined(__GXX_ABI_VERSION) // See PR #5439.
 #        if __GXX_ABI_VERSION >= 1002 && __GXX_ABI_VERSION < 2000
 #            if !defined(_GLIBCXX_USE_CXX11_ABI)
 #                error "UNEXPECTED: _GLIBCXX_USE_CXX11_ABI not defined: PLEASE REVISE THIS CODE."