CI: drop OpenSUSE builds

it turned out to be almost impossible to install libsodium on
OpenSUSE (for example Factory repo url is broken).

Let us drop OpenSUSE builds for a while. Maybe we'll get them back later.

.github/workflows/opensuse.yml

@@ -1,24 +0,0 @@
-name: OpenSUSE
-
-on: [push]
-
-jobs:
-  tumbletweed:
-      name: gcc
-      runs-on: ubuntu-latest
-      container:
-        image: opensuse/tumbleweed
-      steps:
-        - name: Install prerequisites
-          run: |
-            zypper refresh
-            zypper --non-interactive install libopenssl-devel cmake gcc gcc-c++ tar gzip git ncurses-devel zlib-devel readline-devel
-        - uses: actions/checkout@master
-          with:
-            submodules: true
-        - name: Build
-          run: |
-            ./configure
-            make -j $(nproc || sysctl -n hw.ncpu || echo 4) -C build
-            ldd build/vpnserver
-