target/arm/helper.c: Delete stray obsolete comment

In commit 88ce6c6e we switched from directly fishing the number
of breakpoints and watchpoints out of the ID register fields to
abstracting out functions to do this job, but we forgot to delete the
now-obsolete comment in define_debug_regs() about the relation
between the ID field value and the actual number of breakpoints and
watchpoints.  Delete the obsolete comment.

Reported-by: CHRIS HOWARD <cvz185@web.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220513131801.4082712-1-peter.maydell@linaro.org