The code was not allowing to unset a sampler object from a texture,
so the faulty condition was removed to fix the issue.

Fixes 30 tests in:
dEQP.functional.samplers.*

Change-Id: Ibbc3389d3d6135db7bc5a2f87f0df5527b78c7fd
Reviewed-on: https://swiftshader-review.googlesource.com/7360Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>