Re-remove libmagic install from travis config, it's no longer needed (#15157)

.travis.yml

@@ -61,12 +61,6 @@ before_install:
   - mysql -e 'create database olympia;'
   - export GOPATH=$HOME/go
   - export PATH=$HOME/usr/local/go/bin:$GOPATH/bin:$PATH
-  # Unfortunately we require the most recent libmagic-dev version to make use of more recent
-  # JSON detection features which we require for the code-manager related APIs
-  - echo "deb http://archive.ubuntu.com/ubuntu eoan main" | sudo tee -a /etc/apt/sources.list
-  - sudo apt-get update -qq
-  - sudo apt-get install -t eoan libmagic-dev libmagic1 libmagic-mgc
-  - echo "text/markdown                                   md markdown" | sudo tee -a /etc/mime.types
   - sudo cp ./docker/etc/mime.types /etc/mime.types
 
 install: