Fix inconsistency with HeapTuple freeing in extended_stats_funcs.c

Fix inconsistency with HeapTuple freeing in extended_stats_funcs.c

heap_freetuple() is a thin wrapper doing a pfree(), and the function
import_pg_statistic(), introduced by ba97bf9cb7b4, had the idea to call
directly pfree() rather than the "dedicated" heap tuple routine.

upsert_pg_statistic_ext_data already uses heap_freetuple().  This code
is harmless as-is, but let's be consistent across the board.

Reported-by: Yonghao Lee <yonghao_lee@qq.com>
Discussion: https://postgr.es/m/tencent_CA1315EE8FB9C62F742C71E95FAD72214205@qq.com