x86/entry/32: Remove SEP test for SYSEXIT

SEP must be already be present in order for do_fast_syscall_32() to be
called on native 32-bit, so checking it again is unnecessary.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230721161018.50214-6-brgerst@gmail.com