sparc: remove wrong comment from arch/sparc/include/asm/Kbuild

These are NOT exported to userspace.

The headers listed in arch/sparc/include/uapi/asm/Kbuild are exported.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

arch/sparc/include/asm/Kbuild

 # SPDX-License-Identifier: GPL-2.0
-# User exported sparc header files
-
 generated-y += syscall_table_32.h
 generated-y += syscall_table_64.h
 generated-y += syscall_table_c32.h