[N21] Inconsistent use of onlyVm modifier

[asaj]

Description

Share this code via inheriting CalledByVm.sol

Tested

No

Other changes

No

Related issues

• Fixes https://github.com/celo-org/celo-labs/issues/298

Backwards compatibility

Yes

[mrsmkl]

GasPriceMinimum.sol, GoldToken.sol and StableToken.sol still have their own onlyVm modifiers.

[codecov]

Codecov Report

Merging #2903 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2903   +/-   ##
=======================================
  Coverage   73.39%   73.39%           
=======================================
  Files         570      570           
  Lines       14273    14273           
  Branches     1721     1721           
=======================================
  Hits        10475    10475           
  Misses       3513     3513           
  Partials      285      285

Flag	Coverage Δ
#mobile	73.77% <0%> (ø)	⬆️
#web	72.85% <0%> (ø)	⬆️

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