Remove loop detection

[rowanseymour]

Once mailroom starts doing it https://github.com/nyaruka/mailroom/pull/537

[codecov]

Codecov Report

Merging #390 (30f9a91) into main (b609d29) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #390      +/-   ##
==========================================
+ Coverage   71.40%   71.43%   +0.03%     
==========================================
  Files          93       93              
  Lines        8252     8231      -21     
==========================================
- Hits         5892     5880      -12     
+ Misses       1758     1754       -4     
+ Partials      602      597       -5     

Impacted Files	Coverage Δ
backend.go	20.00% <ø> (ø)
backends/rapidpro/backend.go	61.31% <ø> (-0.29%)	⬇️
sender.go	84.31% <ø> (+5.41%)	⬆️
test.go	33.20% <ø> (-0.26%)	⬇️
backends/rapidpro/contact.go	46.15% <0.00%> (-2.57%)	⬇️

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