dev_manager: enabled no_flush for suspend

dev_manager: enabled no_flush for suspend

While the activation code tries to evaluate which target
really needs flush with suspend and which may go without flush,
it has stayed effectively disabled by original commit:
33f732c5e9493cda4b161a18b3d53885d207e3b8 since here
it only allows to pass non-pvmoving  'mirrors'.

So remove check for mirror LV type and only disable
no_flush for 'pvmove'..

TODO: Looking into history - it also seemed like raid target
would have always required flushing but it's been later
removed without clean explanation.

If some more targets really do need 'no_flush' it should
been handle at their 'level' - since we now stack multiple
targets over itself.