Further simplify 'gcc/omp-oacc-neuter-broadcast.cc:record_field_map_t'

Now that I've resolved GCC 'hash_map' issues (a while ago already), we may
further simplify this after commit 049eda82
"Avoid 'GTY' use for 'gcc/omp-oacc-neuter-broadcast.cc:field_map'": as
'hash_map' Value, directly store 'field_map_t' objects, not pointers to
manually allocated 'field_map_t' objects.

	gcc/
	* omp-oacc-neuter-broadcast.cc (record_field_map_t): Further
	simplify.  Adjust all users.