Re-enable single server mode in Drone CI
Drone CI comes since version 1.5.0 with multi-server mode (in which builds are processed by external CI runners) activated by default. This can be problematic if you’re used to drone running in single server mode. If single-server mode isn’t explicitly disabled and no runners are configured it just causes your builds to be stuck with a pending status. It took me quite a while to figure why drone suddenly stopped processing builds (to be fair I updated without reading the changelog)....