drm/ttm: Delete an unnecessary check before the function call "ttm_tt_destroy"

[SF Markus Elfring]

> When resending pls describe what changed (and why).

v3: A bit of reformatting with current software

v2: Broken patch where I managed to delete a "t" too much in a source code
    line somehow.

v1: See also a similar update suggestion
    https://lkml.org/lkml/2015/11/16/416


Would you like to pick such a software adjustment up?

Regards,
Markus