docs: Remove docs for non-existing uppermem command

docs: Remove docs for non-existing uppermem command

Remove all documentation of and mentions of the uppermem
command from the docs/grub.texi file.

The uppermem command is not implemented in the GRUB source
at all and appears to never have been implemented despite
former plans to add an uppermem command.

To reduce user confusion, this even removes the paragraph
describing how GRUB's uppermem command was supposed to
complement the Linux kernel's mem= parameter.

Signed-off-by: Hans Ulrich Niedermann <hun@n-dimensional.de>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>