Fix: remove tracked download resources to avoid erroneous double-releases

[pfrazee]

In my ctzn tests, I had remote hyperbees throw the following error during a hypercore .close():

/Users/paulfrazee/work/ctzn/node_modules/@hyperspace/rpc/index.js:11
    const err = new Error(message)
                ^

Error: Invalid resource: 12628
    at Object.decode (/Users/paulfrazee/work/ctzn/node_modules/@hyperspace/rpc/index.js:11:17)
    at RPC._onmessage (/Users/paulfrazee/work/ctzn/node_modules/arpeecee/index.js:113:33)
    at Parser._nextState (/Users/paulfrazee/work/ctzn/node_modules/arpeecee/parser.js:127:14)
    at Parser._readMessage (/Users/paulfrazee/work/ctzn/node_modules/arpeecee/parser.js:54:19)
    at Parser.recv (/Users/paulfrazee/work/ctzn/node_modules/arpeecee/parser.js:35:44)
    at RPC._write (/Users/paulfrazee/work/ctzn/node_modules/arpeecee/index.js:170:22)
    at WritableState.update (/Users/paulfrazee/work/ctzn/node_modules/streamx/index.js:168:14)
    at updateWriteNT (/Users/paulfrazee/work/ctzn/node_modules/streamx/index.js:479:6)
    at processTicksAndRejections (node:internal/process/task_queues:82:21) {
  code: '',
  errno: 0,
  details: ''
}

I tracked this down to the deleteResource() calls which were releasing active downloads. Turns out completed downloads were not being removed from the set of tracked download-resources, leading to this error due to a double-release.

The PR fixes this by removing a completed download from the set.