This function is unused.

Aside from being unnecessary, on Windows the flag prevents two instances
of Dolphin (one instance from before 2509-371 when the flag was
introduced and the other after) from running the same ROM
simultaneously.

Attempting to do so generated the false error `"[Rom]" is an invalid
GCM/ISO file, or is not a GC/Wii ISO.` followed by `Failed to init core`
and emulation shutdown on the second instance to start the game. Fixing
the incorrect error message is a task I'm deferring to another PR.

The problem didn't happen when both instances were 2509-371 or later,
but I ran into it while bisecting an issue and it'd be nice to avoid
that problem in the future.

Common: Treat DirectIOFile as unable to create SAF files

HookableEvent: Allow hooks to be added and removed from within a Trigger callback.

DolphinQt/QtUtils: Simplify RunOnObject and eliminate Common::Event race. Introduce Common::OneShotEvent class.

In 405baed805, we made the assumption that whether OpenAndroidContent
is able to create a new file depends on the open mode, the document
provider, and the positions of the celestial bodies. However, I'm
fairly sure that it can't create files at all as currently implemented.

First, ContentResolver.openFileDescriptor is documented as throwing
FileNotFoundException if the file doesn't exist. Now, the SAF
documentation is notoriously unreliable on matters like these, and
document providers can do whatever they want anyway, so we can't
actually trust this to mean that FileNotFoundException will always
be thrown if the file doesn't exist. But second, the Dolphin function
ContentHandler.unmangle is also unable to handle files that don't
already exist (unless the passed-in URI isn't mangled to begin with,
but to the best of my knowledge, there's no way to get a non-mangled URI
for a file that doesn't exist (unless you make assumptions about how the
document provider's URI scheme works, which we don't do in Dolphin)).
Summed up, it looks a lot like OpenAndroidContent can't create files,
or at the very least can't do so reliably.

Therefore I'm making DirectIOFile throw an assertion and return false
in situations where it's being asked to create a file under SAF. For
reference, there's no code in Dolphin that actually tries to create a
file under SAF. In all instances where we use SAF to write to files, the
file is created by the system file picker before it returns the URI of
the file to us.

What does this change gain us? First, if someone in the future tries to
use DirectIOFile to create a file under SAF, they'll be immediately
informed that this isn't supported rather than discovering it doesn't
work and chalking it up to SAF being bad for unpredictable reasons.
Second, we get rid of a call to File::Exists, which is a notable
performance improvement for game list scanning due to SAF and Dolphin's
"unmangling" being bad for reasons that unfortunately are entirely
predictable to those of us who have stared into the SAF void.

I've tested that game list scanning and game conversion still works.