flatpak: Drop useless build options

The build options are already defined in the runtime, dconf access isn't
needed anymore, and the build-options flag doesn't seem to be valid.
11 jobs for flatpak-build-options in 15 minutes and 48 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #251283
librem5
api-visibility

00:02:55

passed #251285
librem5
build-debian-clang

00:04:19

passed #251284
librem5
build-debian-gcc

00:03:11

passed #251286
librem5
build-fedora-gcc

00:04:15

 
  Test+Docs
passed #251287
librem5
build-gtkdoc

00:02:28

passed #251288
librem5
unit-test

00:02:25

41.9%
 
  Package
passed #251289
librem5
package-debian-buster

00:02:30

passed #251290
librem5:arm64
package-debian-buster:arm64

00:05:51

passed #251291
flatpak
package-flatpak

00:01:43

 
  Test Package
passed #251292
librem5
autopkgtest-debian-buster-package

00:03:08

passed #251293
librem5
lintian-debian-buster-package

00:01:14