Revert ".travis.yml: Use an LXD container instead of VM"

This reverts commit faa3c3cf5b.

Build took over six minutes for each Node.js version because NVM
had to build each from source. So yeah, let's go back to amd64 VMs.

.travis.yml

@@ -4,8 +4,6 @@ node_js:
   - 12
   - 13
 script: npm run build
-os: linux
-arch: arm64
 dist: bionic
 
 # vim: ts=2 sw=2 et