fix(modem): Fix vfs terminal not to reset read_cb() automatically

This change is needed for the fs terminal to work correctly again, after
merging 44bae24 which shifted
responsibility of thread safety from a terminal to the DTE. (Before it
was the terminal, which removed this callback safely when processing completed,
and thus we had to check the callback return value). Not the DTE layers
are responsible for thread safety and the return value indicates
success/failure from processing.

components/esp_modem/src/esp_modem_term_fs.cpp

@@ -122,9 +122,7 @@ void FdTerminal::task()
         } else {
             if (FD_ISSET(f.fd, &rfds)) {
                 if (on_read_priv) {
-                    if (on_read_priv(nullptr, 0)) {
-                        on_read_priv = nullptr;
-                    }
+                    on_read_priv(nullptr, 0);
                 }
             }
         }