Remove redundant MemoryContextSwith in BeginCopyFrom

Hi, hackers

When I read the code of COPY ... FROM ..., I find there is a redundant
MemoryContextSwith() in BeginCopyFrom().  In BeginCopyFrom, it creates
a COPY memory context and then switches to it, in the middle of this
function, it switches to the oldcontext and immediately switches back to
COPY memory context, IMO, this is redundant, and can be removed safely.

-- 
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.