cmake: remove scary warning

Remove the text saying

"the curl cmake build system is poorly maintained. Be aware"

... not because anything changed just now, but to encourage users to use
it and subsequently improve it.

Closes #5984

CMakeLists.txt

@@ -47,8 +47,6 @@ include(CheckCCompilerFlag)
 
 project(CURL C)
 
-message(WARNING "the curl cmake build system is poorly maintained. Be aware")
-
 file(STRINGS ${CURL_SOURCE_DIR}/include/curl/curlver.h CURL_VERSION_H_CONTENTS REGEX "#define LIBCURL_VERSION( |_NUM )")
 string(REGEX MATCH "#define LIBCURL_VERSION \"[^\"]*"
   CURL_VERSION ${CURL_VERSION_H_CONTENTS})