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>

docs/grub.texi

@@ -1114,12 +1114,6 @@ grub> @kbd{initrd16 /initrd}
 Finally, run the command @command{boot} (@pxref{boot}).
 @end enumerate
 
-@strong{Caution:} If you use an initrd and specify the @samp{mem=}
-option to the kernel to let it use less than actual memory size, you
-will also have to specify the same memory size to GRUB. To let GRUB know
-the size, run the command @command{uppermem} @emph{before} loading the
-kernel. @xref{uppermem}, for more information.
-
 
 @node NetBSD
 @subsection NetBSD
@@ -4030,7 +4024,6 @@ you forget a command, you can run the command @command{help}
 * true::                        Do nothing, successfully
 * trust::                       Add public key to list of trusted keys
 * unset::                       Unset an environment variable
-* uppermem::                    Set the upper memory size
 @comment * vbeinfo::                     List available video modes
 * verify_detached::             Verify detached digital signature
 * videoinfo::                   List available video modes
@@ -5348,12 +5341,6 @@ Unset the environment variable @var{envvar}.
 @end deffn
 
 
-@node uppermem
-@subsection uppermem
-
-This command is not yet implemented for GRUB 2, although it is planned.
-
-
 @ignore
 @node vbeinfo
 @subsection vbeinfo