drop unnecessary import
drop unnecessary import

GHC 8.6.3 warns:

generator/xcffibgen.hs:19:1: warning: [-Wunused-imports]
    The import of ‘Data.Semigroup’ is redundant
      except perhaps to import instances from ‘Data.Semigroup’
    To import instances alone, use: import Data.Semigroup()
   |
19 | import Data.Semigroup
   | ^^^^^^^^^^^^^^^^^^^^^

Signed-off-by: Tycho Andersen <6b523322678240882acbb4da5312a12d8cd0082d@tycho.ws>

tych0/xcffib