Make "Chain Select" mechanism robust against missing 'target' form field

Make "Chain Select" mechanism robust against missing 'target' form field

If removeElement() was invoked on an element that is the 'target' of a
'chainSelect' (which can happen through a form hook in an extension),
the chainSelect handling code was subsequently erroring out.

Add checks to avoid that.

This is related to the CRM-15476 fixes in
8f9c3cbea992320f344d576c8a87c5bab318466c and
14b2ff1521a8c25e363b13723c2979f52ad1cd47 , which already fixed the
situation where the 'control' element is missing.