GitHub Actions: Fix tests; remove unused workflows

.github/workflows/benchmark.yml

@@ -1,28 +0,0 @@
-name: Benchmark
-on: push
-jobs:
-  benchmark:
-    runs-on: [self-hosted, benchmark]
-    if: "contains(github.event.head_commit.message, '[benchmark]')"
-    timeout-minutes: 10080
-    steps:
-    - uses: actions/checkout@v1
-    - name: Benchmark
-      run: |
-        julia --project=@. -e 'using Pkg; Pkg.instantiate()'
-        make build/sysimage.so
-        make -C benchmark clean
-        make -C benchmark -kj4
-        make -C benchmark tables
-        make -C benchmark clean-mps clean-sol
-    - name: Upload logs
-      uses: actions/upload-artifact@v2
-      with:
-        name: Logs
-        path: benchmark/results/*
-    - name: Upload tables & charts
-      uses: actions/upload-artifact@v2
-      with:
-        name: Tables
-        path: benchmark/tables/*
-

.github/workflows/test.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        julia-version: ['1.4', '1.5', '1.6']
+        julia-version: ['1.3', '1.4', '1.5', '1.6']
         julia-arch: [x64, x86]
         os: [ubuntu-latest, windows-latest, macOS-latest]
         exclude: