Unused variables in recent raster code

[robe]

dustymugs,

Assume these are result of your recent changes. Welcome back

Dronie and Gitlab Ci have adopted the same anal retentive mindset as their faster. Soo you must now not have unused variables lying around, lest you get cursed by them and eventually their father may notice and come after you.

rt_raster.c: In function 'rt_raster_gdal_drivers':
rt_raster.c:1721:14: error: unused variable 'is_raster' [-Werror=unused-variable]
  const char *is_raster;
              ^
cc1: all warnings being treated as errors
make[2]: Leaving directory `/drone/src/git.osgeo.org/gogs/postgis/postgis/raster/rt_core'
make[1]: Leaving directory `/drone/src/git.osgeo.org/gogs/postgis/postgis/raster'
make[2]: *** [rt_raster.o] Error 1
make[1]: *** [corelib] Error 2
make: *** [all] Error 1