[skip ci] Delete .ci/Ubuntu20.04 directory

.ci/Ubuntu20.04/Dockerfile

@@ -1,26 +0,0 @@
-FROM ubuntu:20.04
-
-RUN apt-get update && \
-    DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
-        build-essential \
-        ccache \
-        clang-format \
-        cmake \
-        file \
-        g++ \
-        git \
-        liblzma-dev \
-        libmariadb-dev-compat \
-        libprotobuf-dev \
-        libqt5multimedia5-plugins \
-        libqt5sql5-mysql \
-        libqt5svg5-dev \
-        libqt5websockets5-dev \
-        protobuf-compiler \
-        qt5-default \
-        qt5-image-formats-plugins \
-        qtmultimedia5-dev \
-        qttools5-dev \
-        qttools5-dev-tools \
-    && apt-get clean \
-    && rm -rf /var/lib/apt/lists/*