Use xorg-libxt from conda-forge

[jakirkham]

As we provide a package in conda-forge for libXt, switch to it instead of getting from yum.

Checklist

• [x] Used a fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [x] Re-rendered with the latest conda-smithy
• [x] Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[jakirkham]

@conda-forge-admin, please re-render.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-webservice.

I rerendered the feedstock and it seems to be already up-to-date.

[grlee77]

looks good to me

[marcelotrevisani]

I'm going to merge this. Thanks @jakirkham :)