Drop ambiguous MaybeHandle comparison and hashing ops.

Drop ambiguous MaybeHandle comparison and hashing ops.

The default equality comparison operators and hashing functions for
Handles are ambiguous. The intended semantics might have either been
based on Handle locations or on object identity. This is why such
operators do not exist on Handle. The same argument applies to the
MaybeHandle class as well. Comments in that regard were also added.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1319383002

Cr-Commit-Position: refs/heads/master@{#30468}