sysvipc: Remove the linux shm-pad.h file

Each architecture overrides the struct msqid_ds which its required
kernel ABI one.

Checked on x86_64-linux-gnu and some bases sysvipc tests on hppa,
mips, mipsle, mips64, mips64le, sparc64, sparcv9, powerpc64le,
powerpc64, and powerpc.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>