extcon: Remove the deprecated extcon functions

This patch removes the deprecated extcon functions using string type
to identify the type of external connector. The Commit 2a9de9c0


("extcon: Use the unique id for external connector instead of string)
uses the unique id to separate the type of external connector instead of
string name.
- extcon_register_interest()
- extcon_unregister_interest()
- extcon_set_cable_state()
- extcon_get_cable_state()

And, extcon_register_interest() finds the first extcon device to include the
requested external connector from extcon client device and then register the
notifier if extcon device argument is NULL. Instead, extcon_register_notifier()
supports this feature.

But, this patch remains the deprecatd function definition to prevent
the build break.

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