Remove C++ support from the public header

Since BIND 9 headers are not longer public, there's no reason to keep
the ISC_LANG_BEGINDECL and ISC_LANG_ENDDECL macros to support including
them from C++ projects.

fuzz/fuzz.h

@@ -18,7 +18,6 @@
 #include <stdint.h>
 
 #include <isc/dir.h>
-#include <isc/lang.h>
 #include <isc/mem.h>
 #include <isc/once.h>
 #include <isc/types.h>
@@ -26,8 +25,6 @@
 
 #include <dst/dst.h>
 
-ISC_LANG_BEGINDECLS
-
 extern bool debug;
 
 int
@@ -40,5 +37,3 @@ LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
 	if ((x) != ISC_R_SUCCESS) { \
 		return (0);         \
 	}
-
-ISC_LANG_ENDDECLS