libungif: remove and use giflib everywhere

* giflib says it's API and ABI compatible with libungif
  which was in wide use while the LZW compression algorithm was
  patented.

* the selection between giflib and libungif wasn't ever determinictic
  because of conflicts in sysroot
* resolves couple of undeterministic dependencies and:
  WARNING: The recipe giflib is trying to install files into a shared area
  when those files already exist. Those files and their manifest location
  are:
   sysroots/qemuarm/usr/include/gif_lib.h
      Matched in manifest-qemuarm-libungif.populate_sysroot
      Please verify which package should provide the above files.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>