interfaces/builtin: discard empty Validate{Plug,Slot}

[zyga]

The validate methods used to be mandatory but they are entirely optional
now and can be safely discarded.

Signed-off-by: Zygmunt Krynicki zygmunt.krynicki@canonical.com

[codecov-io]

Codecov Report

Merging #3620 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3620      +/-   ##
==========================================
+ Coverage   75.01%   75.09%   +0.08%     
==========================================
  Files         383      383              
  Lines       33208    33176      -32     
==========================================
+ Hits        24910    24913       +3     
+ Misses       6494     6459      -35     
  Partials     1804     1804

Impacted Files	Coverage Δ
interfaces/builtin/i2c.go	67.85% <ø> (+2.33%)	⬆️
interfaces/builtin/iio.go	70.31% <ø> (+2.13%)	⬆️
interfaces/builtin/gpio.go	61.01% <ø> (+2%)	⬆️
interfaces/builtin/docker_support.go	64.28% <ø> (+2.92%)	⬆️
interfaces/builtin/browser_support.go	77.5% <ø> (+7.04%)	⬆️
interfaces/builtin/dbus.go	69.27% <ø> (+1.63%)	⬆️
interfaces/builtin/serial_port.go	67.27% <ø> (+1.2%)	⬆️
interfaces/builtin/mpris.go	67.18% <ø> (+3.95%)	⬆️
interfaces/builtin/bool_file.go	63.63% <ø> (+3.63%)	⬆️
interfaces/builtin/hidraw.go	66.97% <ø> (+1.2%)	⬆️
... and 3 more

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