Revert "i2c: create debugfs entry per adapter"

This reverts commit e2a268b0 which is
commit 73febd77 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I4561926ae6f69310f5217894c6936c1dbfff8c00
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>