gdbserver: remove unused variable

When building with clang 15, I got this error:

  CXX    server.o
server.cc:2985:10: error: variable 'new_argc' set but not used [-Werror,-Wunused-but-set-variable]
  int i, new_argc;
             ^
Remove the unused variable to eliminate the error.

Tested by rebuilding on x86_64-linux with clang 15.