magit-rigid-key-bindings: remove variable

Setting `magit-popup-use-prefix-argument' to `default' has
almost the same effect.  For differences consult this:

key popup                 rigid                    default
----------------------------------------------------------
c   magit-commit-popup    magit-commit             dito
m   magit-merge-popup     magit-merge              dito
b   magit-branch-popup    magit-checkout           dito
M   magit-remote-popup    magit-branch-manager     dito
f   magit-fetch-popup     magit-fetch-current      dito
F   magit-pull-popup      magit-pull               dito
J   magit-am-popup        magit-apply-mailbox      no default
!   magit-run-popup       magit-git-command-topdir magit-git-command
P   magit-push-popup      magit-push               dito
t   magit-tag-popup       magit-tag                dito
l   magit-log-popup       magit-log                dito
o   magit-submodule-popup magit-submodule-update   no default
B   magit-bisect-popup    undefined                no default
z   magit-stash-popup     magit-stash              dito
magit-mode-map: rearrange binding definitions
magit-commit-mode-map: inherit from magit-diff-mode-map

The definition of the latter has to be moved.
magit-jump-to-diffstats: bind in magit-diff-mode-map

instead of in `magit-mode-map'.  Only diff and commit buffers contain
diffstats.  Keep "M-g" bound to it, but also bind "j d" for consistency
with bindings of other `magit-jump-to-{*}' commands.  Also stop using
`kbd' in `magit-diff-mode-map'.
magit-status-mode-map: move definition

right below that of `magit-mode-map'.  Also stop
using `kbd' and rearrange binding definitions.
magit-status-mode-map: absorb magit-section-jump-map
magit-{*}-map: stop using kbd
Merge branch 'n/cleanup-keymaps' into next
magit-clean: new command

Add new command `magit-clean' replacing `magit-reset-working-tree'
which did amalgamate `git reset --hard' with `git clean ...'.  While
former users of the removed command will likely disagree initially,
the old command made very little sense.  Yes, it is likely one would
want to do both things at the same time but one might also want to
only do the cleaning, which was not possible with the old command.