si549: remove 20s warm up

src/libboard_artiq/src/si549.rs

@@ -647,10 +647,6 @@ pub mod wrpll {
         if rc {
             tag_collector::reset();
             reset_plls().expect("failed to reset main and helper PLL");
-
-            info!("warming up refclk...");
-            // refclk need a couple seconds for freq counter to read it properly
-            timer.delay_us(20_000_000);
             set_base_adpll(timer).expect("failed to set base adpll");
 
             // clear gateware pending flag