build: fix nightly flatpak.

Don't set both a branch and a commit, otherwise flatpak-builder will
compare the HEAD of this branch (which may evolve with the commit).

Fixes:

> Failed to download sources: module qoi: Git commit for branch master is dfc056e813c98d307238d35f7f041a725d699dfc, but expected f6dffaf1e8170cdd79945a4fb60f6403e447e020

build/flatpak/org.gimp.GIMP-nightly.json

@@ -785,7 +785,6 @@
                 {
                     "type": "git",
                     "url": "https://github.com/phoboslab/qoi.git",
-                    "branch": "master",
                     "commit": "f6dffaf1e8170cdd79945a4fb60f6403e447e020"
                 }
             ]