gallivm: (trivial) git rid of assertion in float->uint conversion code

Commit 8c3d3622d9ce2fd2a8f46084ab8153d708fa5b09 introduced a new assertion,
but since it causes lp_test_conv failures remove it again and let's hope
we don't really hit bugs caused by the potentially bogus code (it is possible
the assert() caught some cases which work correctly too).