Remove method SplashFTFontFile::getCodeToGID

This method was added in cc43c720
for the use in the ArturOutputDev::updateFont.
It is not use there anymore, and can therefore be removed again.
5 jobs from remove-splash-from-arthur-backend in 11 minutes and 27 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #44158
build

00:04:15

passed #44159
build_clang_libcpp

00:04:08

passed #44162
build_clazy

00:03:37

passed #44160
build_gcc_49

00:08:30

passed #44161
build_mingw64_fedora

00:07:17