plugins/screencast: Remove irrelevant code

testCreateDmaBuf() is going to use the GbmGraphicsBufferAllocator under
the hood.

The GbmGraphicsBufferAllocator works as follows: if you pass it the
DRM_FORMAT_MOD_INVALID modifier, the resulting GraphicsBuffer is going
to have the invalid modifier as well. So the receivedModifiers check
is unnecessary.


(cherry picked from commit 2014f9a3)