libgd: avoid recursive and redundant dependencies

Change the CONFLICTS line from the libgd-full to libgd to fix a
recursive dependency.

While at it, remove the redundant +LIBGD_TIFF:libtiff
+LIBGD_FREETYPE:libfreetype dependencies from Package/libgd/default.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>