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)