Revert "Change mbbisearch to return the character range"

Revert "Change mbbisearch to return the character range"

This reverts commit 78ab944cd4b9977732becd9d0bc83223b88af9a2.

After I had committed eb0d0d2c7 and 78ab944cd, I decided to add
a sanity check for a "can't happen" scenario just to be cautious.
It turned out that it already happened in the official Unicode source
data, namely that a character can be both wide and a combining
character. This fact renders the aforementioned commits unnecessary,
so revert both of them.

Discussion:
https://www.postgresql.org/message-id/CAFBsxsH5ejH4-1xaTLpSK8vWoK1m6fA1JBtTM6jmBsLfmDki1g%40mail.gmail.com