compat/libav: remove unneeded things

Currently, it's unused. Still keep the file, because it's not unlikely
we'll need it again, and removing/readding the include statements for
this file is too annoying.
timer: account for negative time values

It can easily happen that mp_time_us_to_timespec() gets a time in the
past, and then the time difference will be negative. Regression
introduced in commit f47a4fc3.

Also fix an underflow check in mp_add_timeout().