Administration: Make all color schemes available when running from /src.

To run WordPress from /src you have to use the `--dev` flag which also builds the color scheme stylesheets thus the restriction is no longer required.

See #44492.
Fixes #50558.
Built from https://develop.svn.wordpress.org/trunk@48311


git-svn-id: https://core.svn.wordpress.org/trunk@48080 1a063a9b-81f0-0310-95a4-ce76da25c4cd