BeginCopyTo - remove switching to old memory context in between COPY TO command processing

Hi,

While reviewing patch at [1], it has been found that the memory
context switch to oldcontext from cstate->copycontext in between
BeginCopyTo is not correct because the intention of the copycontext is
to use it through the copy command processing. It looks like a thinko
from the commit c532d1 [2]. Attaching a small patch to remove this.

Thoughts?

[1] https://www.postgresql.org/message-id/539760.1642610324%40sss.pgh.pa.us
[2]
commit c532d15dddff14b01fe9ef1d465013cb8ef186df
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date:   Mon Nov 23 10:50:50 2020 +0200

    Split copy.c into four files.

Regards,
Bharath Rupireddy.