[GDBRemote] Remove code that flushes GDB remote packets

The arbitrary timeout when flushing GDB remote packets caused
non-determinism and flakiness between test runs. I suspect it is what's
causing the flakiness of the reproducer tests on GreenDragon, and want
to see if removing it causes that to go away.

This change was originally introduced in r197579 to discard a
`$T02thread:01;#4` that QEMU was sending. If anybody knows how to test
that this continues working after removing this code, I'd love to hear
it.

llvm-svn: 364669