kernel: remove problematic pend() assertion

This assertion, if built in, allows users threads to crash
the kernel in a critical section by passing a negative timeout
value, creating a DoS attack vector.

Remove this assertion, immediately below it there's a check
which just resets it to 0 anyway.

Fixes: #22999

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>