remove feature for automatic jpeg/png/zlib library download and build

this is a post-stable feature and would require a general rewrite of offline processing

cmake/Modules/LAMMPSUtils.cmake

@@ -105,23 +105,6 @@ function(FetchPotentials pkgfolder potfolder)
   endif()
 endfunction(FetchPotentials)
 
-# Check and record if we have a working internet connection
-function(check_for_internet_connection variable)
-  message("Checking internet connection... ")
-  if((CMAKE_SYSTEM_NAME STREQUAL "Windows") AND (NOT CMAKE_CROSSCOMPILING))
-    execute_process(COMMAND ping www.google.com -n 2
-      OUTPUT_QUIET ERROR_QUIET RESULT_VARIABLE PING_STATUS)
-  else()
-    execute_process(COMMAND ping www.google.com -c 2
-      OUTPUT_QUIET ERROR_QUIET RESULT_VARIABLE PING_STATUS)
-  endif()
-  if(PING_STATUS EQUAL 0)
-    set(${variable} TRUE PARENT_SCOPE)
-  else()
-    set(${variable} FALSE PARENT_SCOPE)
-  endif()
-endfunction(check_for_internet_connection)
-
 # set CMAKE_LINUX_DISTRO and CMAKE_DISTRO_VERSION on Linux
 if((CMAKE_SYSTEM_NAME STREQUAL "Linux") AND (EXISTS /etc/os-release))
   file(STRINGS /etc/os-release distro REGEX "^NAME=")

cmake/Modules/Packages/COMPRESS.cmake

@@ -1,10 +1,4 @@
-if(NOT ZLIB_FOUND)
-  find_package(ZLIB)
-  if(NOT ZLIB_FOUND)
-    message(WARNING "zlib library not found skipping COMPRESS package")
-    return()
-  endif()
-endif()
+find_package(ZLIB REQUIRED)
 target_link_libraries(lammps PRIVATE ZLIB::ZLIB)
 
 find_package(PkgConfig QUIET)