[SYCL] Remove unnecessary mutex locks from release and retain functions (#6192)

Commit removes mutex locks from PI release and retain functions. They
were added under impression that 2 threads working simultaneously on
releasing resources can both reach ref count 0 and cause double free.
But since our ref count is atomic and decrement is an atomic operation
only single thread will execute code under if (--(PiObj->RefCount) == 0).
There can't be 2 threads which can reach ref count 0.

Behavior is undefined if DPCPP runtime calls somehow uses deleted pi object
(with ref count 0) or provides it as an argument to any of the pi functions.

Author: againull