ci: remove Drone CI configuration

All build systems now GitHub Actions workflows, so we can remove the
alternative configuration and only keep a single pipeline definition.

.drone.yml

@@ -1,49 +0,0 @@
-kind: pipeline
-name: default
-
-steps:
-  - name: compile
-    image: maven:3-eclipse-temurin-21
-    commands:
-      - ./mvnw -B clean compile
-    when:
-      branch:
-        - main
-        - develop
-        - feature/*
-        - fix/*
-        - release/*
-  - name: unit-tests
-    image: maven:3-eclipse-temurin-21
-    commands:
-      - ./mvnw -B test
-    when:
-      branch:
-        - develop
-        - feature/*
-        - fix/*
-  - name: setup-vault
-    image: alpine:latest
-    environment:
-      VAULT_VERSION: 1.18.0
-    commands:
-      - wget -q -O vault_$${VAULT_VERSION}_linux_amd64.zip https://releases.hashicorp.com/vault/$${VAULT_VERSION}/vault_$${VAULT_VERSION}_linux_amd64.zip
-      - wget -q -O - https://releases.hashicorp.com/vault/$${VAULT_VERSION}/vault_$${VAULT_VERSION}_SHA256SUMS | grep linux_amd64 | sha256sum -c
-      - mkdir -p .bin
-      - unzip vault_$${VAULT_VERSION}_linux_amd64.zip -d .bin
-      - rm vault_$${VAULT_VERSION}_linux_amd64.zip
-    when:
-      branch:
-        - main
-        - release/*
-  - name: unit-integration-tests
-    image: maven:3-eclipse-temurin-21
-    environment:
-      VAULT_VERSION: 1.18.0
-    commands:
-      - export PATH=$${DRONE_WORKSPACE}/.bin:$${PATH}
-      - ./mvnw -B -P integration-test verify
-    when:
-      branch:
-        - main
-        - release/*