Remove this hard-coded buffer size. In some basic experiments preprocessing

Remove this hard-coded buffer size. In some basic experiments preprocessing
large files, this doesn't seem significantly better than just letting
raw_ostream pick a buffer size.

This code predates raw-ostream's automatic buffer sizing; in fact, it
was introduced as part of the code which would eventually become
raw_ostream.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@101473 91177308-0d34-0410-b5e6-96231b3b80d8