kbuild: fix make mrproper for Documentation/DocBook/man

"make mandocs" generate > 2000 files in Documentation/DocBook/man
and this caused kbuild to barf out during make mrproper like this:

make -f scripts/Makefile.clean obj=Documentation/DocBook
make -f scripts/Makefile.clean obj=Documentation/DocBook/man/
make[2]: execvp: /bin/sh: Argument list too long
make[2]: *** [__clean] Error 127
make[1]: *** [Documentation/DocBook/man/] Error 2
make: *** [_mrproper_Documentation/DocBook] Error 2

The man directory were solely used for output
so the fix is to remove it entirely during the
make mrproper process.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>