CI: Remove travis as it is bitrotten.

It needs to be replaced with GHA anyways.

.travis.yml

@@ -1,17 +0,0 @@
-language: c
-script:
-  - make
-  - make -C tests/gadget-zero
-
-addons:
-  apt:
-    sources:
-    - sourceline: 'ppa:team-gcc-arm-embedded/ppa'
-    packages:
-    - gcc-arm-embedded
-
-notifications:
-  irc:
-    channels:
-      - "irc.libera.chat#libopencm3"
-    use_notice: true

README.md

@@ -1,6 +1,5 @@
 README
 ======
-[![Build Status](https://travis-ci.org/libopencm3/libopencm3.svg?branch=master)](https://travis-ci.org/libopencm3/libopencm3)
 
 [![Discord](https://img.shields.io/discord/613131135903596547?logo=discord)](https://discord.gg/P7FYThy)