y2038: alpha: Remove unneeded ipc uapi header files

The alpha ipcbuf/msgbuf/sembuf/shmbuf header files are all identical
to the version from asm-generic.

This patch removes the files and replaces them with 'generic-y'
statements as part of the y2038 series. Since there is no 32-bit
syscall support for alpha, we don't need the other changes, but
it's good to have clean this up anyway.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>