The import of ‘TcType’ is redundant

[philderbeast]

In getting ready for the add to stackage #13, I encountered a build warning;

> git clone https://github.com/adamgundry/uom-plugin.git
> cd uom-plugin
> stack build --test --bench --haddock --resolver lts-11.9
/.../uom-plugin/uom-plugin/src/Data/UnitsOfMeasure/Plugin/Convert.hs:12:1:
warning: [-Wunused-imports]
    The import of ‘TcType’ is redundant
      except perhaps to import instances from ‘TcType’
    To import instances alone, use: import TcType()
   |
12 | import TcType
   | ^^^^^^^^^^^^^

[adamgundry]

Fixed by #45. Thanks!