Remove redundant build-options from the Flatpak manifest

Since fd.o 18.08, i.e. GNOME 3.30, the runtime provides a comprehensive list of build flags, surmounting the current one.
7 jobs for wip/flatpak-remove-custom-build-flags in 25 minutes and 23 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #203460
fedora

00:02:55

passed #203079
ubuntu

00:17:26

failed #203078
fedora

00:14:21

 
  Test
passed #203080
deb-package

00:03:03

passed #203081
flatpak

00:04:30

 
  Deploy
passed #203082
review

00:00:30

failed #203083
allowed to fail manual
stop_review

 
Name Stage Failure
failed
stop_review Deploy There has been a missing dependency failure
No job trace