Remove util.RemoveAll as no longer needed

[zeripath]

util.RemoveAll was a workaround pre-go1.7 which is no longer needed and should have been removed.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@e836b88). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master    #6299   +/-   ##
=========================================
  Coverage          ?   38.86%           
=========================================
  Files             ?      358           
  Lines             ?    51016           
  Branches          ?        0           
=========================================
  Hits              ?    19826           
  Misses            ?    28321           
  Partials          ?     2869

Impacted Files	Coverage Δ
models/admin.go	81.63% <0%> (ø)

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