Remove a special Windows-only call to WinPtyProcess.destroy

This code isn't necessary anymore. The agent process automatically exits
once it flushes the last of the scraped CONOUT/CONERR into its output
pipes (WINPTY_SPAWN_FLAG_EXIT_AFTER_SHUTDOWN). We wouldn't want to kill
the agent before that point, because we'd generally lose data.

Even without WINPTY_SPAWN_FLAG_EXIT_AFTER_SHUTDOWN, it shouldn't be
necessary anymore to kill the agent. Once all (one or both) of the
WinPtyProcess's WinPTYInputStream instances have been closed, the WinPty
object is closed, which calls winpty_free, which kills the agent.

Analogously, with UnixPtyProcess, the pty master FDs are closed when the
input streams are closed.

In theory, I think this code could prematurely close the agent connection
while it still has data in its CONOUT/CONERR NamedPipe output queues. If
that happened, the data would be lost.