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.

include/wx/dynlib.h

@@ -29,9 +29,6 @@ class WXDLLIMPEXP_FWD_BASE wxDynamicLibraryDetailsCreator;
 #elif defined(HAVE_DLOPEN)
     #include <dlfcn.h>
     typedef void               *wxDllType;
-#elif defined(HAVE_SHL_LOAD)
-    #include <dl.h>
-    typedef shl_t               wxDllType;
 #elif defined(__WXMAC__)
     #include <CodeFragments.h>
     typedef CFragConnectionID   wxDllType;