Remove multi-arch config from PR action

.github/workflows/pull-request.yml

@@ -7,10 +7,7 @@ jobs:
     name: Build Bitcoind
     runs-on: ubuntu-18.04
 
-    # NOTE: qemu v3.1.1 used instead of currently newest v4.1.0, because v4 is **much** slower for aarch64, see:
-    #   https://github.com/meeDamian/docker-berkeleydb/commit/9e87d11314c2522726497f0c6059e61a31298e7f/checks
     env:
-      QEMU_VERSION: v3.1.1
       DOCKER_BUILDKIT: 1
 
     strategy:
@@ -22,42 +19,13 @@ jobs:
           - '0.18'
           - '0.19'
 
-        arch:
-          - amd64
-
     steps:
       - uses: actions/checkout@v1.0.0
 
-      - name: Register self-compiled qemu
-        if: matrix.arch != 'amd64'
-        run: docker run --rm --privileged meedamian/simple-qemu:${QEMU_VERSION}-${{matrix.arch}} -p yes
-
-      # Alter `Dockerfile` to reference used architecture/image combos explicitly.  Places changed are:
-      #   * all `FROM` statements (ex. `FROM alpine…` -> `FROM arm64v8/alpine…`)
-      #   * BerkeleyDB `COPY` statement (`COPY --from=lncm/berkeleydb:db-4.8.30.NC` gets suffixed with ex. `-arm64`)
-      # `sed` `--expression`s change it in the following way:
-      #   1st: Matches all occurrences of `FROM alpine`, and injects arch prefix before `alpine`, ex: `arm64v8/alpine`
-      #   2nd: Matches BDB version, and appends "-${{matrix.arch}}" to it (note that `&` represents match).
-      - name: Change Dockerfile to use arch-specific base images
-        if: matrix.arch != 'amd64'
-        run: |
-          CPU=${{matrix.arch}}
-          if [[ "${CPU}" == "arm64" ]]; then
-            CPU="arm64v8"
-          fi
-
-          sed -i ${{matrix.subver}}/Dockerfile \
-            -e "s|^FROM alpine|FROM $CPU/alpine|g" \
-            -e "s|db-4.8.30.NC|&-${{matrix.arch}}|g"
-
-      # NOTE: Don't build 32-bit version of 0.17.1, as it's broken & non-essential:
-      #   https://github.com/bitcoin/bitcoin/pull/15950
       - name: Build Bitcoind
-        if: matrix.subver != '0.17' || matrix.arch != 'arm32v7'
         run: docker build  -t bitcoind  ${{matrix.subver}}/
 
       - name: Print Bitcoind version
-        if: matrix.subver != '0.17' || matrix.arch != 'arm32v7'
         run: |
           docker run --rm  --entrypoint=uname  bitcoind  -a
           docker run --rm bitcoind --version