Remove the use of CLOCK_MONOTONIC_RAW.

[iphydf]

The raw clock isn't subject to NTP adjustments. It may run less
accurately than the regular monotonic clock. I see no reason why raw
would be better for tox than the normal one.

This avoids one piece of OS-specific ifdef'd code.

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This change is 

[codecov]

Codecov Report

Merging #1136 into master will decrease coverage by 1.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1136     +/-   ##
========================================
- Coverage    82.5%   81.4%   -1.2%     
========================================
  Files          82      81      -1     
  Lines       14478   14381     -97     
========================================
- Hits        11958   11711    -247     
- Misses       2520    2670    +150

Impacted Files	Coverage Δ
toxcore/mono_time.c	92.8% <ø> (ø)	⬆️
toxcore/friend_connection.c	85.1% <0%> (-9.5%)	⬇️
toxcore/TCP_server.c	69.5% <0%> (-5.5%)	⬇️
toxcore/net_crypto.c	89.6% <0%> (-3.8%)	⬇️
toxcore/list.c	89.6% <0%> (-3.5%)	⬇️
toxcore/TCP_connection.c	75.4% <0%> (-2.9%)	⬇️
toxcore/onion.c	95.9% <0%> (-2.9%)	⬇️
toxav/msi.c	64.2% <0%> (-2%)	⬇️
toxcore/onion_client.c	93.6% <0%> (-1.6%)	⬇️
toxcore/network.c	82.3% <0%> (-1.1%)	⬇️
... and 7 more

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[robinlinden]

Reviewed 1 of 1 files at r1.
Reviewable status: 0 of 1 approvals obtained