Remove unusable a11y APIs for ClickableSpans.

This change essentially backs out ag/683646, which added an API to click
on ClickableSpans within a TextView. This API has the flaw, however,
that ClickableSpans are not parcelable, so they are not in general
reported to AccessibilityServices. That means that services will have no
idea what they are activating.

Since the API is not usable end-to-end, I'm backing it out before the
API is final.

Bug: 17726921
Change-Id: I541c6218f2899ff67a6b32a13fd9ae6f3b53b3c4