memory: remove unneeded union 'pla'
memory: remove unneeded union 'pla'

The pla union has just one member; it exists to prevent the
member's destructor from being called. However, that member
is trivially destructible, so that doesn't accomplish much.

Simplify the code by replacing the union with its sole member.
Message-Id: <e2972db7090dda0e4fed5b4585c536be97e92dfb@scylladb.com>

dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,dreamsxin/seastar,dreamsxin/seastar,cloudius-systems/seastar