serial: imx: remove unused imx_uart_is_imx* functions

clang with W=1 reports
drivers/tty/serial/imx.c:292:19: error:
  unused function 'imx_uart_is_imx21' [-Werror,-Wunused-function]
static inline int imx_uart_is_imx21(struct imx_port *sport)
                  ^
drivers/tty/serial/imx.c:297:19: error:
  unused function 'imx_uart_is_imx53' [-Werror,-Wunused-function]
static inline int imx_uart_is_imx53(struct imx_port *sport)
                  ^
drivers/tty/serial/imx.c:302:19: error:
  unused function 'imx_uart_is_imx6q' [-Werror,-Wunused-function]
static inline int imx_uart_is_imx6q(struct imx_port *sport)
                  ^
These static functions are not used, so remove them.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20230317205710.1672232-1-trix@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>