cleanup(falco): remove decode_uri as it is no longer used

Signed-off-by: Luca Guerra <luca@guerra.sh>

userspace/engine/falco_utils.h

@@ -52,8 +52,6 @@ void readfile(const std::string& filename, std::string& data);
 
 uint32_t hardware_concurrency();
 
-std::string decode_uri(const std::string& str, bool plus_as_space);
-
 namespace network
 {
 static const std::string UNIX_SCHEME("unix://");