This reverts commit 526798a941.
This will make man filetype not modifiable by default, as it is the
superior behavior in my opinion. More importantly, also make it possible
for a user to modify man filetypes by adding `set modifiable` in
`~/.config/nvim/ftplugin/man.vim` or its equivalent.
ref #11450closes#17595
Co-authored-by: Javier López <graulopezjavier@gmail.com>