Removed .gatsby-highlight selectors

[vzhou842]

The .gatsby-highlight selectors don't do anything because it doesn't have a :global() qualifier. The CSS styles aren't applied because the class gets changed from gatsby-highlight to something else due to CSS Modules:

If you wrap them in :global() qualifiers it works, but the styles seem to not be helpful - they make the prism.js code block off center. Since the selectors weren't doing anything to begin with, I've just removed them in this PR since they're dead code.

[codecov]

Codecov Report

Merging #89 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #89   +/-   ##
=======================================
  Coverage   95.71%   95.71%           
=======================================
  Files          28       28           
  Lines         140      140           
  Branches        8        8           
=======================================
  Hits          134      134           
  Misses          6        6

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