Remove unnecessary MemoryStream allocation from ByteArrayContent (#35005)

Remove unnecessary MemoryStream allocation from ByteArrayContent (#35005)

Constructing a ByteArrayContent (or anything that derives from it, like StringContent) or a ReadOnlyMemoryContent  proactively allocates a MemoryStream that it stores as the _bufferedContent on the base HttpContent.  But the 99.99% use case for ByteArrayContent is as a request payload, and this _bufferedContent isn't used for that.  The only time this would be used is if a developer created a ByteArrayContent and then explicitly got the stream from it, e.g. via GetStreamAsync, in which case we're no worse off by doing the creation lazily.  This PR just deletes the unnecessary code / allocation.