Do not clear axis known position when Z is set to silent

Also removed unused (forgotten) variable.

Firmware/Marlin_main.cpp

@@ -11563,7 +11563,7 @@ if(!(bEnableForce_z||eeprom_read_byte((uint8_t*)EEPROM_SILENT)))
 
 void disable_force_z()
 {
-    uint16_t z_microsteps=0;
+    // uint16_t z_microsteps=0;
 
     if(!bEnableForce_z) return;   // motor already disabled (may be ;-p )
 
@@ -11576,7 +11576,7 @@ void disable_force_z()
     tmc2130_init(true);
 #endif // TMC2130
 
-    axis_known_position[Z_AXIS]=false;
+    // axis_known_position[Z_AXIS]=false;
 }