Remove outdated comment on how to set logtape's read buffer size.

Commit b75f467b 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in the file header comment, so remove the outdated sentence altogether.

src/backend/utils/sort/logtape.c

@@ -47,8 +47,7 @@
  *
  * To further make the I/Os more sequential, we can use a larger buffer
  * when reading, and read multiple blocks from the same tape in one go,
- * whenever the buffer becomes empty.  LogicalTapeAssignReadBufferSize()
- * can be used to set the size of the read buffer.
+ * whenever the buffer becomes empty.
  *
  * To support the above policy of writing to the lowest free block,
  * ltsGetFreeBlock sorts the list of free block numbers into decreasing