curl: remove MANUAL from -M output

... and remove it from the dist tarball. It has served its time, it
barely gets updated anymore and "everything curl" is now convering all
this document once tried to include, and does it more and better.

In the compressed scenario, this removes ~15K data from the binary,
which is 25% of the -M output.

It remains in the git repo for now for as long as the web site builds a
page using that as source. It renders poorly on the site (especially for
mobile users) so its not even good there.

Closes #3587