[doc][llvm-objcopy] Remove redundant COFF-specific sub-heading

The sub-heading used to contain the --only-keep-debug switch as that
switch wasn't implemented for ELF at one point. Since the switch is now
in the generic options section, and there are no other options in this
sub-heading, it is pointless and can be deleted.

llvm/docs/CommandGuide/llvm-objcopy.rst

@@ -210,13 +210,6 @@ multiple file formats.
   The order of wildcards does not matter. For example, ``-w -N '*' -N '!x'`` is
   the same as ``-w -N '!x' -N '*'``.
 
-COFF-SPECIFIC OPTIONS
----------------------
-
-The following options are implemented only for COFF objects. If used with other
-objects, :program:`llvm-objcopy` will either emit an error or silently ignore
-them.
-
 ELF-SPECIFIC OPTIONS
 --------------------