github: Simplify the if: conditions for all chained jobs

As all the other jobs depend on the "prepare" job, it's enough to
keep the condition within the "prepare" job and remove/simplify
the rest.

.github/workflows/test-libcxx.yml

@@ -39,7 +39,6 @@ jobs:
           cat $GITHUB_OUTPUT
 
   test-libcxx:
-    if: (github.event_name != 'schedule') || (github.repository == 'mstorsjo/llvm-mingw')
     needs: [prepare]
     strategy:
       fail-fast: false