stmhal: Remove unnecessary HAL_Delay in readline loop.

The reason for having this delay is to reduce power consumption at the
REPL (HAL_Delay calls __WFI to idle the CPU).  But stdin_rx_chr has a
__WFI in it anyway, so this delay call is not needed.

By removing this call, the readline input can consume characters much
more quickly (before was limited to 1000 chrs/s), and has much reduced
dependency on the specific port.