Remove outdated comment on how to set logtape's read buffer size.

Remove outdated comment on how to set logtape's read buffer size.

Commit b75f467b6e removed the LogicalTapeAssignReadBufferSize() function,
but forgot to update this comment. The read buffer size is an argument to
LogicalTapeRewindForRead() now. Doesn't seem worth going into the details
in the file header comment, so remove the outdated sentence altogether.