doc/man: remove manually written man pages

There is only one page written. Having manually-written man pages duplicates
information with doc comments. Besides, man pages are already generated by
Doxygen.

Signed-off-by: Simon Ser <contact@emersion.fr>
Closes: #156
2 jobs for master in 1 minute and 49 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed build-native-autotools #2336004

00:01:49

passed build-native-meson #2336005

00:00:56