serial: samsung: Remove support for legacy clock code

All Samsung platforms are now using the Common Clock Framework and the
legacy clock code is being removed, so remove related dead code from
samsung-serial driver as well.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>