boards: nucleo_g474re revert pyocd workaround

Pyocd couldn't flash the nucleo_g474re board with the target
stm32g474retx (issue #23351).
Therefore PR #23230 introduced a workaround changing the target
to stm32g474rbtx.

In the meantime this issue was resolved, therefore revert the
workaround.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>