Stop nicing external commands run by the DMRaid module (#788007)

No other commands run by GParted are niced, so stop nicing commands run
from the DMRaid module.

I think nicing of possibly long running file system modification
commands would have made virtually no difference because "nice -n 19"
lowered the CPU priority, but such command would be I/O bound.

History:

Nicing of file system modification commands was added by this commit
from 2006-08-21:
    82e6f6b1
    added nice -n 19, so that all extensive filesystem operations will be

Nicing of DMRaid operations was copied into the DMRaid module when it
was added here in 2009-03-14:
    5865c92d
    Added new class for dmraid support

Nicing was removed from file system modification commands with this
commit from 2013-02-22:
    52a2a9b0
    Reduce threading (#685740)

Bug 788007 - Remove minor bits of legacy from DMRaid module