Do not synchronize on M900

Since the advance factor is computed per-segment in LA15, there's no
need to stop the planner. Allow changing K freely at each segment.

This allows varying quality factors for different filling roles, see:
https://github.com/supermerill/Slic3r/issues/108

During pause/resume/crashdetect or powerpanic K might temporarily be out
of sync when used this way. If this becomes an issue, we might need to
store K for each block, as done for the feedrate.

Firmware/Marlin_main.cpp

@@ -2070,8 +2070,6 @@ static float probe_pt(float x, float y, float z_before) {
     *  K<factor>                  Set advance K factor
     */
 inline void gcode_M900() {
-    st_synchronize();
-
     const float newK = code_seen('K') ? code_value_float() : -1;
 #ifdef LA_NOCOMPAT
     if (newK >= 0 && newK < 10)