Fix magit-insert-untracked-files & 'status.showUntrackedFiles' git config

Commit 1c4136e cleaned up magit-insert-untracked-files to always rely
rely on status.showUntrackedFiles, however it's broken when this
configuration is not set:
in such case 'git config status.showUntrackedFiles' returns nothing, not
the default value as the code expected.

Then git status was run just with "-u", which is equivalent to "-u
all". It's not what the user specified by not defining
'status.showUntrackedFiles'. It should be "-u normal" in such case.

This commit fixes and simplify this by removing the git config retrieval
altogether since git status reads this config, even with the
"--porcelain" flag.

(This git status behavior is a little ambiguous in the man: it says the
output format will be stable "regardless of user configuration", so one
can wonder why the 'status.showUntrackedFiles' user config is not
ignored. But it's about the output *format*, and not the output itself.)