Remove config.warn, Node.warn and pytest_logwarning

[nicoddemus]

Fix #3078

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (features@75e93e5). Click here to learn what that means.
The diff coverage is 93.33%.

@@             Coverage Diff             @@
##             features    #4542   +/-   ##
===========================================
  Coverage            ?   95.76%           
===========================================
  Files               ?      111           
  Lines               ?    24895           
  Branches            ?     2456           
===========================================
  Hits                ?    23841           
  Misses              ?      746           
  Partials            ?      308

Flag	Coverage Δ
#docs	29.43% <23.33%> (?)
#doctesting	29.43% <23.33%> (?)
#linting	29.43% <23.33%> (?)
#linux	95.59% <93.33%> (?)
#nobyte	92.4% <93.33%> (?)
#numpy	93.21% <93.33%> (?)
#pexpect	41.85% <33.33%> (?)
#py27	93.79% <93.33%> (?)
#py34	91.91% <93.33%> (?)
#py35	91.93% <93.33%> (?)
#py36	91.95% <93.33%> (?)
#py37	93.95% <93.33%> (?)
#trial	93.21% <93.33%> (?)
#windows	93.94% <93.33%> (?)
#xdist	93.82% <93.33%> (?)

Impacted Files	Coverage Δ
testing/test_warnings.py	98.81% <ø> (ø)
testing/test_config.py	99.42% <ø> (ø)
testing/deprecated_test.py	100% <ø> (ø)
src/_pytest/terminal.py	91.7% <ø> (ø)
testing/test_assertrewrite.py	83.46% <0%> (ø)
src/_pytest/assertion/rewrite.py	95.6% <100%> (ø)
testing/acceptance_test.py	98.28% <100%> (ø)
src/_pytest/hookspec.py	100% <100%> (ø)
testing/test_pytester.py	87.45% <100%> (ø)
src/_pytest/warnings.py	88.88% <100%> (ø)
... and 7 more

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[nicoddemus]

Fixed now (hopefully)

[nicoddemus]

Ready for additional review/merge

[nicoddemus]

Merging as the failure is unrelated.