Remove various assert(constant) calls

These cause warnings from static checkers, and seem to lead to spurious
GCC7 -Wmaybe-uninitialized.

src/statesync/user.cc

@@ -95,7 +95,6 @@ string UserStream::diff_from( const UserStream &existing ) const
       }
       break;
     default:
-      assert( false );
       break;
     }
 
@@ -131,7 +130,6 @@ const Parser::Action &UserStream::get_action( unsigned int i ) const
   case ResizeType:
     return actions[ i ].resize;
   default:
-    assert( false );
     static const Parser::Ignore nothing = Parser::Ignore();
     return nothing;
   }

src/statesync/user.h

@@ -85,7 +85,7 @@ namespace Network {
     /* interface for Network::Transport */
     void subtract( const UserStream *prefix );
     string diff_from( const UserStream &existing ) const;
-    string init_diff( void ) const { assert( false ); return string(); };
+    string init_diff( void ) const { return string(); };
     void apply_string( const string &diff );
     bool operator==( const UserStream &x ) const { return actions == x.actions; }