[ci] Remove Travis CI

This completes the switch to github actions

.travis.yml

@@ -1,22 +0,0 @@
-language: rust
-cache:
-  directories:
-    - $HOME/.cargo
-
-os:
-  - linux
-  - osx
-
-rust:
-  - nightly
-
-script:
-  - ./prepare.sh
-  - ./test.sh --release
-
-env:
-  global:
-    # Enable backtraces for easier debugging.
-    - RUST_BACKTRACE=1
-    # Reduce amount of benchmark runs as they are slow.
-    - COMPILE_RUNS=2 RUN_RUNS=2