Remove checks for shl_load() from configure/CMake

This function is not used any more since e289eb07e1 (Get rid of
non-POSIX code for loading dynlibs on *nix, 2020-05-13), so don't check
for it and don't defined the corresponding HAVE_SHL_LOAD symbol.

build/cmake/setup.h.in

@@ -984,9 +984,6 @@
 /* Define if you have pthread_attr_setstacksize */
 #cmakedefine HAVE_PTHREAD_ATTR_SETSTACKSIZE 1
 
-/* Define if you have shl_load() */
-#cmakedefine HAVE_SHL_LOAD 1
-
 /* Define if you have snprintf() */
 #cmakedefine HAVE_SNPRINTF 1