[mono-api-html] Make --ignore-changes-virtual less lenient.

[mono-api-html] Make --ignore-changes-virtual less lenient.

As mentioned in commit 789517f8, the intentino of
`mono-api-html --ignore-changes-virtual` is so that when a method is
changed from non-virtual to virtual or override, or from virtual to
override, a "Removed member" message isn't triggered.

However, the previous implementation was too lenient. While *adding*
virtual to the method is fine, *removing* is NOT fine, as that would
be an API breaking change (e.g. all classes which attempt to override
a previously virtual method could no longer do so).

Make the virtual check stricter so that virtual/override to
non-virtual changes are not ignored.