powerpc/64s: Remove old STAB comment

This used to be about the 0x4300 handler, but that was moved in commit
da2bc464 ("powerpc/64s: Add new exception vector macros").

Note that "STAB" here refers to "Segment Table" not the debug format.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220928130912.1732466-1-mpe@ellerman.id.au