Documentation: Remove reference to non-existent "unwind_debug" kernel parm

Signed-off-by: Robert P. J. Day <[e-mail]>
Signed-off-by: Adrian Bunk <[e-mail]>

Documentation/kernel-parameters.txt

···
 	norandmaps	Don't use address space randomization
 			Equivalent to echo 0 > /proc/sys/kernel/randomize_va_space
 
- 	unwind_debug=N 	N > 0 will enable dwarf2 unwinder debugging
-			This is useful to get more information why
-			you got a "dwarf2 unwinder stuck"
-
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 TODO: