linux-user: Remove stale "not threadsafe" comments

linux-user: Remove stale "not threadsafe" comments

In linux-user/signal.c we have two FIXME comments claiming that
parts of the signal-handling code are not threadsafe. These are
very old, as they were first introduced in commit 624f7979058
in 2008. Since then we've radically overhauled the signal-handling
logic, while carefully preserving these FIXME comments.

It's unclear exactly what thread-safety issue the original
author was trying to point out -- the relevant data structures
are in the TaskStruct, which makes them per-thread and only
operated on by that thread. The old code at the time of that
commit did have various races involving signal handlers being
invoked at awkward times; possibly this was what was meant.

Delete these FIXME comments:
 * they were written at a time when the way we handled
   signals was completely different
 * the code today appears to us to not have thread-safety issues
 * nobody knows what the problem the comments were trying to
   point out was
so they are serving no useful purpose for us today.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20220114155032.3767771-1-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>