Remove libtgowt from mantic

[Shengjing Zhu]

This package is only for building telegram-desktop. As telegram-desktop is removed from Ubuntu, this library can also be removed together.

The package is currently stuck in proposed migration, due to missing build on riscv64. Fixing build on riscv64 needs upstream support in its build config, https://github.com/desktop-app/tg_owt/blob/dcb5069ff76bd293e86928804208737e6cee2ccc/src/build/build_config.h#L201

[Steve Langasek (vorlon)]

When filing removal requests, it's appreciated to show the output of 'reverse-depends src:$pkg' and 'reverse-depends src:$pkg -a source' to show you've confirmed there are no reverse-dependencies.

(I've confirmed that it's fine.)

Removing packages from mantic:
 libtgowt 0~git20220508.10d5f4b+dfsg-1build1 in mantic
  libtgowt-dev 0~git20220508.10d5f4b+dfsg-1build1 in mantic amd64
  libtgowt-dev 0~git20220508.10d5f4b+dfsg-1build1 in mantic arm64
  libtgowt-dev 0~git20220508.10d5f4b+dfsg-1build1 in mantic armhf
  libtgowt-dev 0~git20220508.10d5f4b+dfsg-1build1 in mantic ppc64el
  libtgowt-dev 0~git20220508.10d5f4b+dfsg-1build1 in mantic riscv64
  libtgowt-dev 0~git20220508.10d5f4b+dfsg-1build1 in mantic s390x
Comment: New version FTBFS, only useful for telegram-desktop; LP: #2021567
1 package successfully removed.
Removing packages from mantic-proposed:
 libtgowt 0~git20230105.5098730+dfsg-2 in mantic
  libtgowt-dev 0~git20230105.5098730+dfsg-2 in mantic amd64
  libtgowt-dev 0~git20230105.5098730+dfsg-2 in mantic arm64
  libtgowt-dev 0~git20230105.5098730+dfsg-2 in mantic armhf
  libtgowt-dev 0~git20230105.5098730+dfsg-2 in mantic ppc64el
  libtgowt-dev 0~git20230105.5098730+dfsg-2 in mantic s390x
Comment: New version FTBFS, only useful for telegram-desktop; LP: #2021567
1 package successfully removed.

[Steve Langasek (vorlon)]

And blacklisted.