nios2: remove custom early console implementation

As of commits d8f347ba35cf ("nios2: enable earlycon support"),
0dcc0542 ("serial: altera_jtaguart: add earlycon support") and
4d9d7d89 ("serial: altera_uart: add earlycon support"), the nios2
architecture and the altera_uart/altera_jtaguart drivers support
earlycon. Thus, the custom early console implementation for nios2 is no
longer necessary to get early boot messages. Remove it and rely fully on
earlycon support.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>