Remove OpenCL warper

- Even when OpenCL was detected at build time, this has been disabled
  at runtime by default due to multiple issues
- Performance improvements weren't that big compared to multithreaded
  CPU
- Wasn't functionaly on par with CPU version
- Nobody cared enough about it to maintain it
- Big users of GDAL (remote sensing companies) don't care that much
  about GPU uage of GDAL.

.github/workflows/cmake_builds.yml

@@ -55,7 +55,7 @@ jobs:
            libpoppler-dev libpoppler-private-dev gpsbabel libhdf4-alt-dev libhdf5-serial-dev libpodofo-dev poppler-utils \
            libfreexl-dev unixodbc-dev libwebp-dev liblcms2-2 libcrypto++-dev libkml-dev \
            libmysqlclient-dev libarmadillo-dev wget libfyba-dev libjsoncpp-dev libexpat1-dev \
-           libclc-dev ocl-icd-opencl-dev libsqlite3-dev sqlite3-pcre libpcre3-dev libspatialite-dev libsfcgal-dev fossil libcairo2-dev libjson-c-dev libdeflate-dev liblz4-dev libblosc-dev libarchive-dev \
+           libsqlite3-dev sqlite3-pcre libpcre3-dev libspatialite-dev libsfcgal-dev fossil libcairo2-dev libjson-c-dev libdeflate-dev liblz4-dev libblosc-dev libarchive-dev \
            libqhull-dev libcfitsio-dev libopenjp2-7-dev libheif-dev \
            python3-dev libpython3-dev libpython3.10-dev python3.10-dev python3-numpy python3-lxml python3-pyflakes python3-setuptools python3-pip python3-venv \
            python3-pytest swig doxygen texlive-latex-base make cppcheck ccache g++ \

.github/workflows/ubuntu_20.04/Dockerfile.ci

@@ -74,8 +74,6 @@ RUN apt-get update -y \
     make \
     mono-mcs \
     numactl \
-    ocl-icd-opencl-dev \
-    opencl-c-headers \
     openjdk-8-jdk-headless \
     pkg-config \
     python3-dev \

alg/CMakeLists.txt

@@ -82,12 +82,6 @@ if (GDAL_USE_ARMADILLO)
   gdal_target_link_libraries(alg PRIVATE ${ARMADILLO_LIBRARIES})
 endif ()
 
-if (GDAL_USE_OPENCL)
-  gdal_target_link_libraries(alg PRIVATE OpenCL::OpenCL)
-  target_compile_definitions(alg PRIVATE -DHAVE_OPENCL)
-  target_sources(alg PRIVATE gdalwarpkernel_opencl.h gdalwarpkernel_opencl.cpp)
-endif ()
-
 gdal_target_link_libraries(alg PRIVATE PROJ::proj)
 
 if (GDAL_USE_QHULL_INTERNAL)