Remove MIPS32 accept4, recvmmsg, sendmmsg implementations.

MIPS32 has its own implementations of accept4, recvmmsg and sendmmsg
because at one point it needed to avoid socketcall being used for
those functions (MIPS32 has socketcall, but has never used it in
glibc, and so never had socket.S at the time when socketcall used such
a per-architecture file instead of C code).  The current code no
longer uses socketcall based on __NR_socketcall being defined, and the
syscalls are always present on MIPS for supported kernels so the
socketcall case in the code is dead for MIPS; this patch removes the
implementations that are, as Adhemerval noted, no longer needed.

Tested compilation for mips-linux-gnu with build-many-glibcs.py.

	* sysdeps/unix/sysv/linux/mips/mips32/accept4.c: Remove file.
	* sysdeps/unix/sysv/linux/mips/mips32/recvmmsg.c: Likewise.
	* sysdeps/unix/sysv/linux/mips/mips32/sendmmsg.c: Likewise.