Revert "WebUI: explicitly specify GITHUB_BASE for pixel test commands"

This reverts commit dc9b3a5.

When pulling pixel test references with the full link GITHUB_BASE is not
needed. This would be needed if we would use only the PR number.

ui/webui/test/README.rst

@@ -139,11 +139,11 @@ Locally just copy the broken tests images to the `test/reference` directory. How
 option to deal with this is to use automation which will download all the broken images from
 fail test on PR::
 
- GITHUB_BASE=rhinstaller/anaconda ./test/common/pixel-tests fetch <link to HTML with failed tests>
+ ./test/common/pixel-tests fetch <link to HTML with failed tests>
 
 Example of such a call::
 
- GITHUB_BASE=rhinstaller/anaconda ./test/common/pixel-tests fetch https://cockpit-logs.us-east-1.linodeobjects.com/pull-4551-20230322-101308-479c2fc1-fedora-rawhide-boot-rhinstaller-anaconda
+ ./test/common/pixel-tests fetch https://cockpit-logs.us-east-1.linodeobjects.com/pull-4551-20230322-101308-479c2fc1-fedora-rawhide-boot-rhinstaller-anaconda
 
 The link will be link accessible from the `Details` button on GitHub PR with failed tests.