This reverts commit 6012145.

The `source` field in IO::Buffer can have a String or an IO::Buffer
object, if not nil.

- When the `source` is a String object. The `base` field points to the
  memory location of the String content, which can be embedded in
  RSTRING, and in that case, GC compaction can move the memory region
  along with the String object.

  Thus, IO::Buffer needs to pin the `source` object to prevent `base`
  pointer from becoming invalid.

- When the `source` is an IO::Buffer, then `base` is a pointer to a
  malloced or mmapped memory region, managed by the source IO::Buffer.
  In this case, we don't need to pin the source IO::Buffer object,
  since the referred memory region won't get moved by GC.

Closes: [Bug #21210]