contrib: remove "thunderbird-patch-inline"

The "thunderbird-patch-inline" directory in "contrib/" contains a script
to send patch files via Thunderbird. This script depends on the
ExternalEditor extension [1], which seems to be effectively unmaintained
with the last update being in 2008. While the extension has eventually
been maintained in [2], that fork hasn't received any updates since
2020, either.

As such, the ExternalEditor extension does not work with modern versions
of Thunderbird anymore, and as the "thunderbird-patch-inline" script
depends on the ExternalEditor extension it likely doesn't work anymore,
either. The fact that this script hasn't been touched for the last 10
years outside of some global cleanup supports the idea that it is not
useful anymore.

Remove it.

[1]: https://globs.org/articles.php?lng=en&pg=2
[2]: https://github.com/exteditor/exteditor/releases

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t