glsl: remove unreachable assert()

Earlier commit enforced that we'll bail out if the number of terminators
is different than 2. With that in mind, the assert() will never trigger.

Fixes: 56b86739 ("glsl: fix infinite loop caused by bug in loop
unrolling pass")
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 8eceac9d)