mpu6000: remove factory test

[julianoes]

My assumption is that this is not used or needed and we can remove it to save some flash space.

     VM SIZE    
 -------------- 
  [ = ]       0    [Unmapped]
  [ = ]       0    [section .debug_abbrev]
  [ = ]       0    [section .debug_aranges]
  [ = ]       0    [section .debug_frame]
  [ = ]       0    [section .debug_info]
  [ = ]       0    [section .debug_line]
  [ = ]       0    [section .debug_loc]
  [ = ]       0    [section .debug_ranges]
  [ = ]       0    [section .debug_str]
  -6.0%     -20    mpu6000_main
  [DEL]     -44    mpu6000::factorytest()
  -0.1%     -64    [section .text]
  [DEL] -1.20Ki    MPU6000::factory_self_test()
  -0.1% -1.33Ki    TOTAL

[LorenzMeier]

I wouldn't want to strip the code (we do need self-tests and will add more of them) but I think this could be guarded behind the same low-flash flag that we're using in other places.

[julianoes]

@LorenzMeier I agree but we don't yet have such a define, or I can't find it.

Ok, I've added a define. It's not pretty.

[dagar]

This is fine for now as it was effectively unused. The line I don't want to cross with CONSTRAINED_FLASH is any actual functionality change from a users perspective.

In the new drivers I was planning to add factory self test as part of init, but haven't gotten around to it yet.