help.c: Remove "kmem -l" help messages

"kmem -l" option has existed when crash git project initialization, but
its help message was not accurate (extra arguments a|i|ic|id was missing).

In addition, those symbols required by the -l option were for very old
kernels, at least 2.6 kernels don't contain them.  Also, this option has
not been fixed for a long time.

Instead of document this option, hide it from help messages.

Signed-off-by: Li Zhijian <[email protected]>

help.c

@@ -6888,8 +6888,6 @@ char *help_kmem[] = {
 "            members of the associated page struct are displayed.",
 "   address  when used with -c, the address must be a page pointer address;",
 "            the page_hash_table entry containing the page is displayed.",
-"   address  when used with -l, the address must be a page pointer address;",
-"            the page address is displayed if it is contained with the list.",
 "   address  when used with -v, the address can be a mapped kernel virtual",
 "            address or physical address; the mapped region containing the",
 "            address is displayed.\n",