ibm_storage: Remove unused _get_os_type method
ibm_storage: Remove unused _get_os_type method

platform.dist() doesn't exist on Python 3.8+, so this
no longer works.  This method is not called by any
other code.

Change-Id: I97b000ad8eb2bf63aaa0bb110be6ca39b8359a94

openstack/cinder,mahak/cinder,openstack/cinder,mahak/cinder