Update ubuntu.yml to remove ubuntu 18.04

actions/runner-images#6002
18.04 actions runner image was deprecated

Author: XinfengZhang

.github/workflows/ubuntu.yml

@@ -47,40 +47,3 @@ jobs:
         make -j$(nproc)
         make check
         sudo make install
-
-  ubuntu-18-04:
-    runs-on: ubuntu-18.04
-    steps:
-    - name: checkout libva
-      uses: actions/checkout@v3
-      with:
-        repository: intel/libva
-        path: libva
-    - name: checkout libva-utils
-      uses: actions/checkout@v3
-      with:
-        path: libva-utils
-    - name: install prerequisites
-      run: |
-        sudo apt-get update
-        sudo apt-get install -y --no-install-recommends \
-          libdrm-dev \
-          libegl1-mesa-dev \
-          libgl1-mesa-dev \
-          libx11-dev \
-          libxext-dev \
-          libxfixes-dev \
-          libwayland-dev
-    - name: build libva
-      run: |
-        cd libva
-        ./autogen.sh --prefix=/usr --libdir=/usr/lib/x86_64-linux-gnu
-        make -j$(nproc)
-        sudo make install
-    - name: build libva-utils
-      run: |
-        cd libva-utils
-        ./autogen.sh --prefix=/usr --libdir=/usr/lib/x86_64-linux-gnu
-        make -j$(nproc)
-        make check
-        sudo make install