Combine deletion phase into single recursive function

Similar to the previous step, this converts the deletion phase into
a single recursive function. Although there's less code, this one is
a bit trickier because it's already contains some stack-like logic
for tracking the nearest host parent. But instead of using the actual
stack, it repeatedly searches up the fiber return path to find the
nearest host parent.

Instead, I've changed it to track the nearest host parent on the
JS stack.

(We still search up the return path once, to set the initial host parent
right before entering a deleted tree. As a follow up, we can instead
push this to the stack as we traverse during the main mutation phase.)