Is there anyway to get rid of the 12 character Xbox one gamertag and keep the 15 character...

Once you change your gamertag within the 12 character policy, there is no way to remove it and go back to only the 15 characters.

The 12 character cap is to keep the full limit with the suffix to 15-16 characters.

Why do gamertags only support 12 characters now?

We require at least 3 characters to auto-assign you a suffix and make sure that everybody has an equal chance to have a name they can be proud of. Many older games allowed 16 characters for displaying gamertags.
We’re reserving three of those characters for the suffix and one for the # symbol, leaving 12 characters for gamertags.

Yup, it’s only accepting gamertags that are 12 characters or less. Mine is 15 characters so hopefully this gets fixed soon.

You technically still have the 15 character limit. The gamertag that appears on the system and apps is 12 characters, yes, but you're able to change your gamertag for games not supporting the new gamertag change. You should see

a prompt to customize your gamertag. The one you're changing when selecting said prompt will be the gamertag that appears in games and can go up to 15 characters.

If you change your gamertag on a Xbox 360 you can still have the 15 character limit but other than that you are limited to 12 characters.