libgomp.c/target-map-zero-sized-3.c: Fix code for non-USM offload [PR120530]

A mapping clause was missing, causing the code to fail with offloading
when a host pointer was not device accessible.

libgomp/ChangeLog:

	PR target/120530
	* testsuite/libgomp.c/target-map-zero-sized-3.c (main): Add missing
	map clause; remove unused variable.

(cherry picked from commit 16c742e1079e838b920a1b215af17828da7c6365)