Signed-off-by: Jeff K

As noted by @snowzach in #3461

Also removes the re-definition of now inside M190's while loop

Since residency_start_ms is -1 when entering the heatup while loop whilst the hotend temp is close to the target them already then it may not escape.

Hence "Start the TEMP_RESIDENCY_TIME timer when we reach target temp for the first time."

This may fix a subtle bug caused by doing `G29` more than once without
`G28` between.