extcon: Remove optional print_name() function pointer of extcon_dev

This patch removes the optional print_name() function pointer included in
'struct extcon_dev' because the extcon must maintain the consistent name of
extcon device on sysfs instead of inconsistent name. After merged patch[1],
extcon can maintain the consistent name of extcon device without any hard-coded
device name.
[1] https://lkml.org/lkml/2015/4/27/258

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>