Delete zlib.cmake (#13467)

Delete the file because it is not included by any other file.

cmake/external/zlib.cmake

@@ -1,47 +0,0 @@
-include (ExternalProject)
-
-set(zlib_INCLUDE_DIR ${CMAKE_CURRENT_BINARY_DIR}/external/zlib_archive)
-set(ZLIB_URL https://github.com/madler/zlib.git)
-set(ZLIB_BUILD ${CMAKE_CURRENT_BINARY_DIR}/zlib/src/zlib)
-set(ZLIB_INSTALL ${CMAKE_CURRENT_BINARY_DIR}/zlib/install)
-set(ZLIB_TAG 50893291621658f355bc5b4d450a8d06a563053d)
-
-if(WIN32)
-  set(zlib_STATIC_LIBRARIES
-      debug ${CMAKE_CURRENT_BINARY_DIR}/zlib/install/lib/zlibstaticd.lib
-      optimized ${CMAKE_CURRENT_BINARY_DIR}/zlib/install/lib/zlibstatic.lib)
-else()
-  set(zlib_STATIC_LIBRARIES
-      ${CMAKE_CURRENT_BINARY_DIR}/zlib/install/lib/libz.a)
-endif()
-
-set(ZLIB_HEADERS
-    "${ZLIB_INSTALL}/include/zconf.h"
-    "${ZLIB_INSTALL}/include/zlib.h"
-)
-
-ExternalProject_Add(zlib
-    PREFIX zlib
-    GIT_REPOSITORY ${ZLIB_URL}
-    GIT_TAG ${ZLIB_TAG}
-    INSTALL_DIR ${ZLIB_INSTALL}
-    BUILD_IN_SOURCE 1
-    DOWNLOAD_DIR "${DOWNLOAD_LOCATION}"
-    CMAKE_CACHE_ARGS
-        -DCMAKE_BUILD_TYPE:STRING=Release
-        -DCMAKE_INSTALL_PREFIX:STRING=${ZLIB_INSTALL}
-	-DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=ON
-)
-
-# put zlib includes in the directory where they are expected
-add_custom_target(zlib_create_destination_dir
-    COMMAND ${CMAKE_COMMAND} -E make_directory ${zlib_INCLUDE_DIR}
-    DEPENDS zlib)
-
-add_custom_target(zlib_copy_headers_to_destination
-    DEPENDS zlib_create_destination_dir)
-
-foreach(header_file ${ZLIB_HEADERS})
-    add_custom_command(TARGET zlib_copy_headers_to_destination PRE_BUILD
-    COMMAND ${CMAKE_COMMAND} -E copy_if_different ${header_file} ${zlib_INCLUDE_DIR})
-endforeach()