Remove beta toolchain from CI

Since we never leave Nightly failing for long, this is exceedingly
unlikely to catch any issues not caught by the other toolchains.

.builds/beta.yml

@@ -1,42 +0,0 @@
-image: archlinux
-packages:
-  - rustup
-sources:
-  - https://github.com/ruma/ruma
-tasks:
-  - rustup: |
-      # We specify --profile minimal because we'd otherwise download docs
-      rustup toolchain install beta --profile minimal
-      rustup default beta
-  - test: |
-      cd ruma
-
-      # We don't want the build to stop on individual failure of independent
-      # tools, so capture tool exit codes and set the task exit code manually
-      set +e
-
-      cargo test --all --quiet
-      test_exit=$?
-
-      # ruma-identifiers has a bunch of features. Make sure it works both with
-      # all of them and none of them being enabled.
-      pushd ruma-identifiers
-
-      cargo test --no-default-features --quiet
-      id_test_1_exit=$?
-
-      cargo test --all-features --quiet
-      id_test_2_exit=$?
-
-      popd
-
-      # ruma-client_api also has a few optional features, but none are enabled
-      # by default. Make sure it works with all of them.
-      pushd ruma-client-api
-
-      cargo check --all-features --verbose
-      client_api_exit=$?
-
-      popd
-
-      exit $(( $test_exit || $id_test_1_exit || $id_test_2_exit || $client_api_exit ))