gdb: remove declaration of deprecated_target_wait_hook

Remove the declaration of deprecated_target_wait_hook; the actual
definition and usage were removed 4 years ago in:

  commit fb85cece22a2cb3c0185e61cfc1323e9c5a6466e
  Date:   Sat Mar 12 13:41:47 2022 +0100

    Replace deprecated_target_wait_hook by observers

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