Revert "Add riscv64 support for flake.nix"

This reverts commit f163e8d.

We don't support riscv64 in hydra. This needs to be refactored to only
enable riscv64 in the flake but not in hydra jobs.

flake.nix

@@ -6,7 +6,7 @@
   outputs = { self, nixpkgs }:
 
     let
-      supportedSystems = [ "x86_64-linux" "i686-linux" "aarch64-linux" "riscv64-linux" ];
+      supportedSystems = [ "x86_64-linux" "i686-linux" "aarch64-linux" ];
       forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
 
       version = nixpkgs.lib.removeSuffix "\n" (builtins.readFile ./version);
@@ -81,12 +81,10 @@
               [ self.hydraJobs.tarball
                 self.hydraJobs.build.x86_64-linux
                 self.hydraJobs.build.i686-linux
-                # FIXME: add aarch64/riscv64 emulation to our github action...
+                # FIXME: add aarch64 emulation to our github action...
                 #self.hydraJobs.build.aarch64-linux
-                #self.hydraJobs.build.riscv64-linux
                 self.hydraJobs.build-sanitized.x86_64-linux
                 #self.hydraJobs.build-sanitized.aarch64-linux
-                #self.hydraJobs.build-sanitized.riscv64-linux
                 self.hydraJobs.build-sanitized.i686-linux
                 self.hydraJobs.build-sanitized-clang.x86_64-linux
               ];