[gdb/build] Fix gdbserver/linux-aarch64-low.cc build

Commit 0ee25f97 ("Fix regression on aarch64-linux gdbserver")
removed the last use of i in gdbserver/linux-aarch64-low.cc
(aarch64_target::low_stopped_data_address). Breaking the build on
aarch64 with:

gdbserver/linux-aarch64-low.cc: In member function ?virtual CORE_ADDR aarch64_target::low_stopped_data_address()?:
gdbserver/linux-aarch64-low.cc:557:12: error: unused variable ?i? [-Werror=unused-variable]
  557 |   int pid, i;
      |            ^
cc1plus: all warnings being treated as errors

Fix this by removing the variable i completely.

Fixes: 0ee25f97 ("Fix regression on aarch64-linux gdbserver")