Remove Travis-CI control file

Since we're using GitHub Actions for CI builds this file is no longer
needed.

.travis.yml

@@ -1,87 +0,0 @@
-dist: xenial
-# osx_image: xcode12.2
-
-cache: ccache
-
-language: cpp
-
-os:
-  - linux
-  ## disabled:  - osx
-
-compiler:
-  - gcc
-  ## disabled:  - clang
-
-env:
-  - |
-    ANALYZE=false
-    DOC=false
-
-jobs:
-  include:
-    os: linux
-    ## compiler: clang
-    ## env: ANALYZE=true DOC=true
-
-branches:
-  only:
-  - master
-  - branch-1.3
-  - /^feature-.*/
-  - /^fix-.*/
-  - /^issue-.*/
-  - /^str-.*/
-
-before_script:
-  - |
-    if [ $TRAVIS_OS_NAME = linux ]; then
-       sudo apt-get update -q
-       sudo apt-get install -y libxinerama-dev libxcursor-dev libasound2-dev libglew-dev
-       sudo apt-get install -y doxygen
-    elif [ $TRAVIS_OS_NAME = osx ]; then
-      # brew install ccache # need to install on macOS
-      # export PATH="/usr/local/opt/ccache/libexec:$PATH"
-      # brew install doxygen
-      echo "macOS build is currently disabled"
-    fi
-
-script:
-  - |
-    if [ $ANALYZE = "false" ]; then
-      echo
-      echo "start configure/make at `date`"
-      echo
-      autoconf
-      ./configure
-      make
-      echo
-      echo "start CMake/make at `date`"
-      echo
-      mkdir cmake-build
-      cd cmake-build
-      cmake -G "Unix Makefiles" -D FLTK_BUILD_EXAMPLES=ON ..
-      make
-    else
-      echo
-      echo "start scan-build/make at `date`"
-      echo
-      mkdir scan-build
-      cd scan-build
-      scan-build cmake -G "Unix Makefiles" -D FLTK_BUILD_EXAMPLES=ON ..
-      scan-build -v make
-    fi
-    echo
-    echo "done at `date`"
-    echo
-
-after_success:
-  - |
-    if [ $DOC = "true" ]; then
-      doxygen --version
-      cd documentation
-      make html
-      cd -
-    fi
-
-# use default email notification: committer and author get notifications