Remove arm64 build from Drone

because it started to run on Travis.

.drone.yml

@@ -1,36 +1,3 @@
----
-kind: pipeline
-type: docker
-name: arm64
-platform:
-  os: linux
-  arch: arm64
-steps:
-  - name: test
-    image: ruby:2.5-stretch
-    commands:
-      - uname -m
-      - apt-get -yq update
-      - apt-get -yq install software-properties-common
-      - apt-get -yq install bison sudo
-      # workaround ipv6 localhost
-      - ruby -e "hosts = File.read('/etc/hosts').sub(/^::1\s*localhost.*$/, ''); File.write('/etc/hosts', hosts)"
-      # create user
-      - useradd --shell /bin/bash --create-home test && chown -R test:test .
-      # configure
-      - /usr/bin/sudo -H -u test -- bash -c 'autoconf && ./configure --disable-install-doc --prefix=/tmp/ruby-prefix'
-      # make all install
-      - /usr/bin/sudo -H -u test -- make -j$(nproc) all install
-      # make test
-      - /usr/bin/sudo -H -u test -- make test
-      # make test-spec
-      - /usr/bin/sudo -H -u test -- make test-spec
-      # make test-all
-      - /usr/bin/sudo -H -u test -- make test-all
-trigger:
-  branch:
-    - master
-
 ---
 kind: pipeline
 type: docker